Wasn't there a huge anti-trust lawsuit against Microsoft for preventing other browsers on their platform? How does Apple get away with it when MS couldn't?

Probably a mix of Apple not having a monopoly in the mobile space and other browsers till being allowed on the app store.

Allowing other browsers to be installed didn't stop Microsoft from losing an anti-trust suit.

I am not up to date. I didn't realize they allow other browsers now. When I was building iOS apps Apple had specific wording which prevented apps in the app store which duplicated functionality already available in iOS. That meant things like the AdBlock Firefox build could not be published in the App Store.

It was, in great part, their legal team and strategy that pushed me away from Apple.

Apple still doesn't allow true browsers in the app store. Not because they disallow duplicate system applications (plenty of calculator apps out there). The problem is that Apple does not allow any developer to use a browser engine other than built-in Safari. Apple also doesn't allow virtual machines/on the fly compilation of code, making any alternate, fast enough Javascript engine practically impossible.

"Browsers" on iOS, such as Firefox Focus and Chrome, just repurpose the built-in Safari engine with some modifications (bookmark sync, extra content blocking, etc.)

I think the simplest answer is that no one has sued them for it yet.