Because they affect the way the bit applies pressure and affect the skull differently than a standard bridle. The bit is held in a different position. It is an innovative design, and is a refreshing attempt at trying to create a more ergonomic, horse-friendly bridle.
Me? I don’t like them. They apply more pressure on the poll (bad) than standard cavessons do (the pressure applied on the poll correlates DIRECTLY with how tight the noseband is, BTW- and for Micklems, you have to have the noseband very secure to get the rest of the bridle to function properly) and I also see them adjusted and fitted incorrectly constantly, which IMHO does more harm than good.
I’ve handled a couple Micklems, and their quality is ALL over the board - some are cheap, cardboady feeling and others are more appropriate for the price tag.
I tell most people who ask if it is a good fit for their horse - unless their horse anatomically needs a physically different cavesson (IE, maybe rubs or sarcoids on the face, extreme cheek sensitivity, etc) it isn’t worth the money. They are very similar to drop nosebands in terms of pressure and affect - if people are deadset on trying the Micklem, I usually suggest trying a drop/hanoverian noseband first. Can you tell I worked at a tack store and dealt with Mickelm RAs constantly?!
Anecdotally, every horse I’ve tried in a Micklem has hated them - but they were sensitive TBs (one particularly in the poll region).