Java, NT, .NET, "wide" C and C++ and a few others from the same time frame ended up with WTF-16 because surrogate pairs didn't exist when they were designed. They were designed with UCS-2, which is a fixed-length encoding. Unicode 2.0 then extended that to be variable length (16/32-bit) using surrogate pairs and that's where all the systems come from which don't validate surrogate pairs.
"Wide" characters in C and C++ do not have a standardized width. On most non-Windows platforms they are actually four bytes so can cover the entirety of Unicode and then some in one wchar_t. Not that that's a more reasonable encoding for most applications than WTF-16.
