I believe the included decompiler predates the one that goes with IDA Pro, so that could be a motivation. IDA Pro itself is really really old, having once been

I believe the included decompiler predates the one that goes with IDA Pro, so that could be a motivation. IDA Pro itself is really really old, having once been a 16-bit DOS shareware program, which explains why IDA Pro doesn't use the GUI shortcuts that were standardized much later. IDA Pro still doesn't support collaboration, although there are very broken hacks that attempt to add it. Binary Ninja supports collaboration if you buy the enterprise edition.