Not totally true. My department and at least a few others here in SF Bay Area had MP5's as long guns available to patrol.
The Remington 870 was available to everyone, but if you went through a class and qualified annually you could check out a MP5 (our patrol guns have the N trigger group) at the beginning of shift and check it in at the end.
At my department those that cared carried them and those that didn't care did not or chose a shotgun.
I'd say we had a steady number of folks that carried them up until 2007ish or so. Same with SWAT. By 2008 or 2009, everyone was carrying AR's. Our MP5's and shotguns basically sit.
Having spent substantial time with both the MP5 and AR, I will always choose an AR.