There's probably many complicated reasons, but I'll try to cover the basics.>The engineering for it hasn't been worked out yet
Even two-cans-attached-via-cable imposes complications to assembly and docking. Not saying that the issue is making spinning space stations impossible, it's just an issue that no-one has worked out the subtle nuances yet.>There's no immediate need
The growth of more space stations is glacial at best. Once there are more people spending more time in space, then there will be the need for spinning stations, but right now there isn't a need.>There's no immediate interest
There are very few players in the aerospace industry who are both interested in space stations AND can make one. Most of the players who meet those criteria are only interested in the ISS and LOP-G.
And that last one is the crux of the issue. The players who are only interested in the ISS and LOP-G aren't really into those projects for their cutting edge technology. They're mainly interested in securing funding for their agencies and no more. Why do you think the ISS spent 30 years relearning how bad micro-g is despite Mir doing all of that earlier?
Hopefully this will all change in a couple of years. I'm going to be really disappointed if I die without anything big happening in aerospace. And I mean Apollo big, not some probe that'll putt around an area less than a Midwest highway town in the span of 5 years.