Before you ask why a Chat client grows faster, it's simple:

The onboarding experience is almost friction-free.

There are no random dick picks in new people's DMs. There is no need to find interesting people to follow before making things usable. You don't even need to think about what's interesting to you. There is no need to setup Zaps and there is no need for NIP-05. They are not even going to find political takes they don't like just to be angry about in the first 5 minutes of using the app.

Most of the time, new users don't have anything to say publicly. But there is always something to be said in private. That is how you get them going.

If DM clients really want to make things complicated, they should guide the user into choosing a DM relay, like inbox.nostr.wine. But the rest is just install and message your friend. It couldn't be simpler.

Sure, this only works because the other Nostr apps keep the social structure active. If you were to release a whole new protocol just for chats (outside of Nostr), you would face tremendous resistance. But here the Nostr network effects will work in your favor.