Has PixelFed done something different that Mastodon hasn't?
I suspect image browsing is less reliant on specific content creators, so Pixelfed probably has an advantage there compared to something like Mastodon. You're only going to move to Mastodon if the people you wanna follow are there too, while if you're for example just looking to browse pictures of food and cats you don't really care who is posting so you're not as reluctant to try Pixelfed.
I don't use either Instagram or Pixelfed though so might be totally off base here.