It’s pretty simple really. Activision approach SD [and Mad Doc Software - Wils] to make a commercial expansion pack for RTCW involving both SP, MP and MP bots. The SP and bot development doesn’t go to schedule and falls behind or doesn’t live up to expectations, so the expansion is decided to be MP only. Activision don’t want to pull the plug because they most likely coughed up SD’s wages up front and so want something in return, even if that is only going to be public acclaim. And of course, how better to make the most of this public acclaim than to make the game standalone. Now everyone can enjoy the wonders that SD/Activison have provided.
So in reality, ET was a publishing cock-up that benefitted the public.