There are many stories about why there is no SoI.
Some say save files got corrupted, some say they just couldn't keep it up anymore.
It is hard to say the yogscast is still at their best, especially now they seem to pump out a lot of the same *cough GMOD *cough* type of videos out lately.
I still enjoy watching them but it gets harder and harder for me to follow all of their videos, not because i am superdumb, but because of how much they pump out lately and because they seem to become less and less interesting.