This is just an educated guess, but it was too much info for comments. Maybe someone can come up with a specific answer, but it seems it's probably a certification delay. The CRJ's all use the Rockwell Collins Pro Line 4 avionics suite, which uses CRT's. In 2000 Rockwell Collins introduced the Pro Line 21 suite which uses LCD screens. They have been slowly introducing PL21 retrofits for aircraft with PL4 as they receive STC's. The Falcon 50 and 2000 began getting retrofits in 2008, followed by Beechjet 400A in 2009, King Airs in 2010, Learjet 60 and Gulfstream G100 in 2012, Piaggio Avanti in 2013, then Hawker 800 and Challenger 300 & 605 in 2014.1
My presumption is that the CRJ's have a type cert with the PL4 and they very well may be working on getting a cert with the PL21 or the newer Pro Line Fusion suite. It will certainly become increasingly difficult to find replacement CRT's as there a only a couple of companies still manufacturing them. It seems just swapping display types is not feasible with the PL4 system. The PL21 system was designed to be more flexible so it's easier to integrate improvements in technology.2
1 These dates might be wrong. I'm finding different dates from different sources. For example, Falcons began getting them in the US in 2006 but it didn't receive EASA certification until 2014.