I have noticed the same issue. It's especially bad when there's really promising looking Tutorial and the files ain't there anymore. Why this happen? Well, I can only make some guesses.
Perhaps some people have given up game making (or at least C2) entirely? Or perhaps they have run out of room at the shared disk and cleaned up the oldest stuff.
I know that I have linked here couple test capx that had no other intention than to be a quick help to those asking various question. In a sense, those have already filled their purpose, but I think I'll leave them on in case someone later sees the thread.
At the same time.. I'd like to see some administrative cleanup done at tutorials + scirra arcade.. Especially the arcade has plenty of pure **** in it. I can't talk for Scirra (and don't know their intentions) but IMHO those 'games' give really bad impression of the potential C2 has.