When reading the recent clarifications on drop rate modifications (nerfs) to lower hpt trash mobs, it seems obvious that they have just nerfed the hell out of high MP farming strategies. Act 3 is still the best farming location for mob density, but most of the mobs you would kill when doing high MP runs have had their drop rates cut in half with the recent changes. At high MPs there are also many problems with the 'trash mobs' in other Acts, for example in Act 1 you now run the risk of running into a room full of those club wielding mobs that will 1 shot you through 500 All res/40k+ hpt, whereas in Act 3 you only really had to worry about breaking through a door and taking an exploding fallen maniac to the face.
Act 3 was not 'too good' of a farming spot before these changes, they should have buffed drop rates instead of nerfing them since the vast majority of legendary drops are useless anyway.