It is a temporary restriction to prevent people from relying on impls for large arrays on a stable compiler just yet.
In the rare chance that we need to give up on const generics and rip it out of the compiler, it avoids getting into a situation where users are already relying on e.g.  IntoIterator for [T; 262143]  and we have no way to support them.
When const generics are finally stabilized the restriction would be removed and the impl would apply to arrays of arbitrarily large size.