Comment by dotancohen

I don't know the actual reason on this launch, but leaving stuff up there is not uncommon. On a test launch such as this, they might not want to relight the second stage engine too many times. They also may not have navigation down pat. Better leave one large bird up there where it's easily tracked, than to possibly have it explode leaving lots of smaller debris. Or to try to deorbit it and hit London.

There are tens, if not hundreds, of discarded second stages orbiting Earth right now.