He was responsible for the the events of QOS which took place in 2006 but then left bond alone until the events of Skyfall in 2012?
This is for the same reason that had plagued Eon for decades. They did not have the rights to include Blofeld and Spectre until 2013.
(It is a long story.)
but within the fictional CraigBond Universe...
I think he said he was "the author of all your sorrows" or some such thing. He somehow pulled the strings that caused all the tragedies this particular version of Bond experienced, from Vesper's death through to M's.
How he did this is the big question, as these events involved a lot of unpredictable contingencies, and surely there were easier ways to torment his "foster brother".
a more sensible explanation is he lied about the "author of all your sorrows" thing, as he is shown to be a mindgamer. These events did all happen by chance, and Blofeld is taking credit just to make Bond paranoid.
if so that returns us to the original question of why he wasn't tormenting Bond all this time. I would guess he was busy doing normal everyday evil SPECTRE work until Bond recognised him in that meeting in Rome, then he had to do something.