With shared_ptr you completely lose track who actually owns an object or when it will get destructed. It encourages sloppy programming and makes code much harder to read and reason about.

Unlike most languages that have ref-counting build in, shared_ptr also doesn't provide anything to deal with cyclic dependencies, so you can end up with memory leaks.

The most important reason however is simply that you don't need it like 99% of the time, unique_ptr provides enough functionality to work just fine as a shared_ptr replacement in most situations. And in the rare cases where you really need a shared_ptr, you can just convert a unique_ptr into one.