It's just bloody inefficient to get things done in! Coding in it requires girnormous amounts of text shovelling, typing attempts to be strict, template meta-programming requires hand-coding type inference schemes, and on top of that, the library/package management is a nightmare.
Well, I didn't think these harsh things when I 'left' C++, but as I worked with more high-level languages, I realized that I was doing more, with less code and dynamic duck-y typing, and with nice libraries only a `cpan` away.
So when I hacked together a C++ program a year or so ago, I got punched in the face by all these issues. It was a pain. So I said, okay, this is stupid. I need to use C for low-level work like drivers, and use Common Lisp for other things. Like what everyone else does[1].
Fundamentally, C++ has a number of flaws, of course - that's typical for a pragmatic language - but the key flaw in my opinion is that it's a pain to get higher-level stuff done in until you build the libraries that other language constructs/libraries give you out of the box.
[1] For a very small subset of everyone else.