OpenStack is complex
-> versions of public cloud stacks that require less effort to manage
Exactly. We looked at OpenStack. We had a small test setup running to dip our toes, to see what it is like. The obstacle, call it the "problem" if you like, is you need people who either know OpenStack already, or are prepared to put in the time to learn it. There's nothing wrong with that, but it is a significant commitment right at the start. It is not just another bunch of servers running something which is already well understood.
I'm not surprised that the growth is in the big companies that can afford (the time rather than just the money) to dedicate people to OpenStack.