It has nothing to do with the integration difficulties.

Many ++ devs, me included, come from the C background and the amount of obfuscation Boost adds to the code is simply not worth any benefits it brings along. Yes, you can do a lot with a single line of Boost'ed code, but when it maps onto pages of simpler code, you basically lose a lot of control over the code. It also bloats the binary, which again may not even register as a viable concern with some, but which does have several long-reaching complications.