I think the reason is the existence of move/copy constructors, i.e. it's invalid to just move a C++ object in memory (as a realloc would do), in general.
(shrink_to_fit does need to (attempt to) get a smaller allocation, otherwise or wouldn't be shrinking the vector.)
That is, when you call shrink_to_fit, it first copies all elements it has, swaps itself with the new copy, and deletes the original vector. (Probably because there's no portable way of returning only part of a contiguous memory region. Yikes.)
http://stackoverflow.com/questions/2695552/how-to-shrink-to-...