Footwork depends on rulesett.
You can compare the footwork from boxing or thai/kick boxing, with mma. Different rules. And different thing to worry about.
Another thing is that MMA is a very young sport compared to mma and boxing. So it will take some time before mma got its own high level standard for its techniques.
Lastly. Think of training time. The human body only got so many training hours. A boxer will train only boxing. While a mma fighter got to split it into boxing, kick boxing, wrestling (both standing and on the ground), submission grappling, striking on the ground..
So the MMA fighter dont have the same amount of time to really finetune his footwork.
And even the high level guy with amazing footwork from other sports (like Israel adesanya or gokan Saki), have to change it when converting to mma.
Sorry for the wall of text.