In the JavaScript/TypeScript ecosystem, libraries like NextAuth still have a huge number of users for the same reason: ease of use. And with the rise of full-stack TypeScript apps where both the frontend and backend live together and share a strong type system, it makes even more sense to keep all your context in one place.
That said, if you ever decide to self-host Better Auth in a dedicated container, you still can.
Back when I was looking at it a couple of months ago, the big thing that popped out was that BetterAuth supports email and password out of the box, where NextAuth seems to have a preachy disclaimer about how email and password is inherently insecure, so they leave you to your own devices to implement password hashing and the like.
That did give a sense that NextAuth was the first to dominate the space and feels as though they can dictate morals.
BetterAuth seems to be a bit more developer-focused.
Yeah I needed a login & password auth last friday and I was so frustrated with NextAuth I ended up using nginx to set up http basic auth.