Why would devs do the hard thing and pour their effort into getting something working at the protocol level thats already available by some popular proprietary nostr app? Especially when they have the majority of the users and they are happy with the solution.

The very few people doing protocol dev will stop doing it. Users will be like: wow you got something working in a janky way which we've already had two years ago on NostrGram, who cares. It's becoming not very rewarding work.

You could have easily built zaps in a way that worked only on one app and all the users used that app because its an awesome feature. Building zaps was non-trivial and required lots of work between many different parties. It's sad we're moving away from this way of building things.