don’t think too much into it.
those were the 70s and 80s and the stallions were used because they were “available”.
“available” at the nearby rhineland breeding stations of the westfalian state stud.
BR’s dam is an 1982 horse. back then, breeders would make use of stallions in the local area (life coverage).
the state stud director dr.lehmann would make sure to provide for a decent mixture of jumper/dressage stallions, plus potential refinery stallions at any given local station in westfalila and the rhineland.
cacir and carrera were the refiners at that local station at that time, thus people would make use of them.
you would find the same structure of stallion mixture at any other hannoverian or westfalian state stud breeding station back then. reason you find certain “nick breedings” more often arising form certain local state stud stallion combinations (i.e. weltmeyer brentano in ankum/hann, later on to be continued by lodonderry weltmeyer brentano))
however, in the case of this pedigree, the answer is simple.
frühlingsball was a multi talented sire and probably one of the most successful breeding sires of his time (life coverage). he is known to be a jumper sire because of the local combination of pilot-sons in that area (pit by pilot porbably the most succesful and common combination back than). the rhineland was a jumper area for that reason, but depending on the individual cross, frühlingsball was an excellent dressage sire, too. there are superb dressage marelines descending o/o frühlingsball mares.
(both my fidermark mares have frühlingsball as a damsire and i consider that a real plus since that is where the power from behind and back comes from. just that i wasn’t aware of it when i bought both of them at foal age, i was just lucky and pretty uneducated …).
however, frühlingsball was a heavy sire and his daughter often could use refinery. (both my two fidermark frühlingsball mares are perfect examples as they are completely different, too. the elder one is a real “light” producer, fidermark-type, while the younger one - fannie mae- still re-produces frühlingsball visibly in most combinations, if refinery blood is missing).
cacir AA in third generation to BR was “available” at that local station as a refiner at that time.
in retrospective, a mediocre sire. he didn’t destroy much, but he didn’t add specific supremecy, either.
after all, he did refine, specially when meeting on further refinery blood (carrera).
carrera was a super typey stallion, again, of mediocre performance. beautifil horses, not that much of lasting heritage, though.
frühlingsball sure was the plus in this combination. no surpise, BR’s breeders bred two licensed sires from frühlingsball (franciso I+II), none of them of lasting influence, though. BR’s dam is a full sibling to those two stallions.
check out the damline on horse telex, you can see, they also bred licensed sires by fidermark o/o one of their cacir x frühlingsball mares (fiderglanz I&II). fidermark was available nearby, too. (useful to know, since his semen wasn’t the best, thus, rhineland breeders did take a lot of profit from the nearby availability of the sire).
i’ld say belissimo was the first “hype” stallion in this cross, finalising the area of life coverage. up until than the breeder family relied on breeding BRs dam to jumper stallions (mainly pilot sons, pit and prominenz, the first a nearby state stud son, the latter a nearby private stallion).
they were inflamed like everybody else by belissimo and gave it try - just like everybody else. a sire that was not owned by the state stud (yet, available nearby) and everybody talked about him after his first public appearance at the elite auction age three, where he sold for decent record money of 176.000, still deutschmarks, i believe, maybe first euros, not sure (that time record money, anyway, the local newspaper was all over it).
even hannoverian breeders made use of him, maybe more than westfalian breeders, given his bolero bloodline, and hannover was anxious to find a sire to carry on the bolero heritage.
bolero was big in hannover but really lacking a meaningful son. brentano was too oldfashioned and already “outdated” for most of the breeders at that time. belissimo was beautiful (completely UN-like bolero’s usual sons in hannover), a stamp of his damsire romadour II, who was another excellent stamp stallion, both, jumping and dressage (rembrandt/nicole upphoff’s olympic mount) - and beauty.
ever since, westfalian belissimo is the stamp stallion to maintain the bolero blood, even in hannover.
BR descends from belissimo’s first breeding year and i remember having seen her at the elite show age three. a very unspectacular horse and many said, she only made it elite since the sponsor of isabell werth had already bought her and was printed as the owner in the catalogue.
obviously, isabell had already spotted qualities in her, noone else had 
there is a half sibling by quaterback, the only other dressage combination BRs dam was bred to, but he failed to deliver expected dressage excellence, instead was shown in shows young horse jumping classes age five before he dissapeared from the sport scene.
you can tell, breeding is a very unpredictable business, even for top-breeders.
at that time, when IW acquired BR, the damline had just proven to be a multi talented performance line, “buch” by pit o/o franziska (by frühlingsball) was considered a rising star in the jumping circus and the combination of pit (again, a nearby available pilot-son of excellent jumper genetics) and frühlingsball produced some excellent jumpers (speaking of nick breedings of local state stud stallions). … and another two licensed sons o/o this specific damline, promotion and paladium JMen.
isabell also acquired emilio from that breeders family, not sure at what age, though. he is by ehrenpreis (ehrentusch) and two years younger than BR and descends o/o another cacir x frühlingsball-sibling, so very closely related to BR and the ehrentusch-line, again, is a super performance line. heavy old blood and both, jumping and dressage, depending on how they were crossed. but they do require refinery, reason why many people laughed at isabell when she started emilio. old fashioned, heavy. but he sure works.
i have been visiting the BRs breeders family a few times. today they breed with lesser mares and -like everyone else- do not make use of nearby-stallions anymore but select, what looks promising and popular.
fail&fortune, just like everybody else.