I don't fully agree with this: while you'll never find an item with 70 or 100 MR, as you can with armor, magic resistance is more common, and while
is not an easy early game purchase, Magic Resist items are cheap to fully build from the mid game onwards.
The problem with MR stacking is that getting too much of it too easily shuts down most AP champions: unlike AD champions, who mostly specialize in DPS and can just keep going until their victim dies, most AP champions rely on a burst rotation to deal most of their damage, after which they become fairly weak. Force of Nature allowed tanks and bruisers to ignore most of a mage's burst, regenerate what health they lost, and power through, making mages practically useless (mages did not have a good time in Season 2). While players have a lot of different options to mitigate magic damage, if they want to have enough MR to completely shut down the enemy mage or AP assassin they're going to have to dedicate a lot of item slots for it (by contrast, you just need a few armor slots to get tons of armor, but it doesn't entirely stop the enemy marksman from killing you, given enough time).
Negatron Cloak is also being reinstated on some items in the PBE (namely,
and the upcoming ZZ'Rot Portal). The main reason it was removed in the first place was due to the price and MR hike on Null-Magic Mantle: what with it giving +25 MR from the start, and mid-tier MR items like
,
and
giving between 30 and 40 MR (so not that much more), there wasn't really a need to include yet another middle item for those recipes.