IMO, John Venus hits some key points: predators aren't interested in fair fights, they stalk prey. Whedon probably didn't see a viable victim in that whole bunch, and had a lot less power versus the boys upstairs compared to when he was making his Buffy-verse shows. Not so much Angel, but BtVS was something of - not only a surprise hit - a cultural phenom at the time; college classes were being taught about its impact on English language. I imagine he enjoyed some leverage with the suits that just wasn't there with Marvel Studios, let alone Disney.