The problem is that comptime implies a full compile-time evaluation, while when doing an inline for you only want to unroll over a list of potentially heterogeneous elements but the body of the for loop will remain available for evaluation at runtime, if not all vales are comptime known. In a comptime block that would cause a compile error.

I have an example of that in this blog post: https://kristoff.it/blog/what-is-zig-comptime/