I think he mentioned this briefly in the article. In C it's common to use intrusive data structures like linked lists and AVL trees because that's how the language works, you can embed data structures easily. This wouldn't work with a B tree because it's allocation pattern is very specific. Rust on the other hand is very pro-generics where you can write a B-tree and painlessly have it store whatever anybody wants it to. It's more about what the language makes easy and idiomatic than anything.