Grinding in the studio doesn't equal hard work. Wayne spends a lot of time in the studio too, banging out repetitive records and just throwing verses on beats. From what I've heard Bishop was just in there doing the same shyt over and over, was not growing as an artist, wasn't taking advice, and when it came time to put in work with Dre he wasn't holding his own.
Snoop has a history of being super lazy in the studio, but Dre puts up with it because he recognized Dre+Snoop is always a winning combo. If he doesn't believe in you, he won't put up with that shyt breh. Same reason he doesn't do much with 50 anymore - they've gone in opposite directions musically and 50 has little interest in diversifying outside of gangster pop rap