Diabetus brought it up on his Formspring. Basically:
They thought that their ElectricalBeast videos were too old, and that a good chunk of their fanbase came in after that, so a lot of people wouldn't have seen the prerequisite videos so as to get any jokes a third video would be making. Basically, the optimum time to do the video has long passed.
The conclusion to his Mario 64 LP was really really boring
They should have gotten George Wood on this.
"I had a challenge for Slowbeef. Develop a scenario in which they award me something for suggesting the Lara Croft breast cancer plot. Imagine the drama of a disbelieving slowbeef who, despite his doubts, still persisted in giving me this award. It seems double minded, no pun intended, but I guarantee the internet will be stunned, shocked and amazed at slowbeef's attempt at being nice to make my victory more meaningful. We love Retsupurae, but its about time the industry had a big shock for a change."
I don't know if he was kicked out or if he just distanced himself from SA.
Beef and Betus disliked him early on because of his overuse of catchphrases and "wacky" commentary. Though with the likes of PewDiePie and Tobuscus nowadays they've pretty much moved on from him. I think they wished him well for his wedding on Retsutalk (while still making fun of him).
He made Let's Plays that weren't very good and ignored all criticism, and would throw a fit about people posting videos showing off cool things in areas he already covered (he said they were trying to hijack his thread or something). He made his videos to appeal to his subscribers.
This thread was pretty much the last thing he did at SA. In summary, DC makes a terrible LP thread, people criticize it, and he closes it. Later he makes a new thread with the exact same OP and videos, which leads to predictable results from everyone. (The thread is also coincidentally where slowbeef is more or less chosen to be a mod).
But Deceased Crab is ~4 years irrelevant now. Time to move on to complaining about other people like Pewdiepie.