It has 20 years of organic growth and includes a lot of lessons learned from the initial underspecification. Lots of profiles were specified but never used (video delivery, over bluetooth, anyone?). Many were not fully thought through and had to be revised (often in nasty ways to remain backwards-compatible). Sadly backwards compatibility makes it worse and worse every year. If you go and write a stack that complies with the spec 100%, you'll find that it works with maybe 50% of devices out there.

BT 4.0 was a chance to start over, and it will help a lot, in time... BT 5.1 will likely finally bring stereo audio over BT LE, with a cleaner spec than A2DP is. But once again, until all the EDR (classic) devices out there are trashed, BT classic remains a huge pain in everyone's rear and a mandatory thing to support. How soon is that? More than 10 years from now! Why? Cars are a major BT use case and many still ship with BT 2.1 car kits. People do not replace cars as easily as headphones.

source: i work on this currently