Why is it not the default?

Helix primarily follows the Kakoune model instead of Vim's. It can be confusing to switch from Vim, but Helix's model tends to be more intuitive if one does not already have Vim muscle memory.

Because it's not the same as vim, they've made some fundamentally different choices and their default binds reflect that. I genuinely think helix stands to be better, it lacks the community and thus the extensions of vim/neovim as well as decades of experience, so it's hard to replace it, but for me it's managed to nevertheless.