from the megathread (scroll down to the later half of the post) summarizes pretty well why stall has a very very difficult time working right now - you can't get the right mix of utility and bulk on the team. stall has the need for a LOT of utility roles - you need hazards, you need phazing, you generally need a spinner - and the mons that can do these things are lacking in the bulk needed to survive against this meta's offensive threats. offense teams, depending on how heavily offensive they are, don't generally need as many roles to be filled (offense does not need a phazer at all for example). conversely the things that do have the bulk and survivability tend to be crappy team players that can contribute relatively little to a team besides their name and face - guys like slowbro and amoonguss and bronzong that really don't do much besides the occasional random use of status and pivoting through attacks.
stall has only six slots and it's nigh impossible to fit all of these utilities onto the team without crippling the bulk of the team. in addition stall has to worry a lot more about type stacking than offense does (offense can actually benefit from running multiple mons of the same type - for stall, more than two of a single weakness rapidly becomes a liability) and it really shows when you're so crammed for slots.
in addition there are threats in bw2 whose counter lists are extremely restricted (the lists of true counters to genesect and tornadus-T, for example, are rather short). that means that in order to survive in bw2 you basically have to run certain mons. the sun genesect+dugtrio core is really a prime example of this - if stall loses its genesect counter while the little metal bastard is still alive, genesect takes out any remaining sun counters (eg latias) and you lose. the number of variants of heatran that can beat the core singlehandedly is infamously small - shed shell restalk (we all know how shit it is but sometimes you just don't have the choice), flame charge (lol no), magma storm (shit accuracy and trapping doesn't always fit on the team, especially in stall) or scarf (generally unsuited to stall). the problem is that it can sometimes be even more restrictive to implement a plan across your entire team to deal with these threats, so you have to choose between sacrificing a slot right away for shed shell tran or making a shaky solution on the spot to a premier-class metagame threat.
the list of shit like this goes on and on for stall so it is really really hard right now. weatherless stall is basically impossible (i have tried). rain and sand can stall as always but it's still much harder than making or playing offense teams under those weathers