Why now?
Blooming obvious. The price of these machines has fallen through the floor.
General purpose laptops have also fallen, but not through any price sensitivity threshold. They fell through that threshold about a year or two ago and everyone who wanted one bought then. They're not going to throw out the old, and buy a new one now even though the price continues to fall. There's no advantage in a new one over a 2yo machine.
Now the "certified workstation" grade machines have also dropped low enough that a whole raft more people will feel like buying these.
IDC should hopefully (maybe I'm being too hopeful here) realise that once the gamers have their "certified" kit, they've blown their allowances and won't be buying again for a couple of years.
Who are all the PC makers going to sell to in the interim?