The E145's and E135's regularly use jetways in Milwaukee, with 6 of 14 jetways set up with the RJ adapter bridge. However as the number of large-jet flights has increased, the large majority of RJ flights are ground level boarding. At peak times there's only room for a couple of RJ's to use jetways.
At slower times, they still tend to put RJ's at the ground-boarding gates even if jetways are open. A couple reasons for this:
(a) Not all jetways are RJ-capable.
(b) It you put an RJ at a jetway and there is an irregular ops situation (weather, mechanical, ATC delay, etc) that jetway is unavailable until the RJ can leave. There are only so many jetways, and A319/E190/E170's must use a jetway.
(c) Ramp teams cover sets of gates, so in smaller banks they try to cluster planes together for more efficiency. During smaller banks...especially banks which are primarily RJ...they could put an RJ over on a jetway like 45. But if it's the only flight on the hammerhead, it's not an efficient use of rampers.
(d) The ground-boarding "20's" gates have three consolidated gates, which means multiple planes can be better served by fewer gate agents. Commonly they put two agents at a gate which might serve three parking spots, so it's not like there's normally one agent serving six departures. But most conventional jetway gate positions serve only a single aircraft. So when an RJ is at a gate like 43, either you have two agents serving just a couple of dozen passengers (not very efficient) or a solo agent who can only do one thing at a time...answer the phone, help a customer, go down the jetway to find the F/A, pull boarding passes, etc. Not the best for helping customers.
So while it would be nice to have more RJ's using jetways, there are reasons why the majority don't.