Having used a Sand Stall team extensively, I can definitely attest to how much harder it is to utilize well in B/W2. Given how offensive the metagame is right now, it really isn't catered towards Stall. Also the new introductions B/W2 gave us, including Tornadus-T, Keldeo, Thundurus-T and Genesect make Stall that much harder to play. It has come to the point where there is simply too much to deal with, and sometimes those 6 teamslots are not enough to make your team solid, or to the point where you can successfully wall a large portion of the metagame. Even stall has had to adapt to this, as Stall definitely appreciates even a Scarf 'mon as a catch-all backup to things such as CM Keldeo and NP Thundurus-T. Stall is on the receiving end of the incredible power creep, and it's getting increasingly harder to pull off.
For those talking about stall being risky, of course it's risky. It's probably the most predictable playstyle, as when playing stall you really cannot afford to lose one Pokemon early on in a match leaving you open to something else. There's nothing stopping your opponent double switching into his Sheer Force Landorus predicting your Tentacruel switch, not giving you opportunities to spin agianst offensive teams that work so well by stacking hazards. The influx of Deoxys-D teams also really doesn't help Stall as a playstyle out either, as Deoxys-D is pretty great at guaranteeing hazards while also being able to prevent them at the same time. Another notable mention of a factor against Stall is the introduction of Shadow Tag Gothitelle. Honestly, you can slap Gothitelle on pretty much any team and it works. Specs versions pretty much trap everything on standard stall and remove them, making you so easily prone to Gothitelle's accompanying sweepers. The fact that stall has to check so many different threats means you might not carry 2 answers for one Pokemon, and if Gothitelle removes your Terrakion counter, then it's highly likely you'll be on the receiving end of a Terrakion sweep.
Just looking at the usage stats, which are the only way of seeing what people are using, it's pretty easy to see why stall is suffering so badly. There's simply too much to cater for, and the harsh reality is you only get 6 Pokemon to wall an entire metagame. Even looking through the RMT forum, I don't think I've seen one successful stall team as of yet. I guess it just goes to show that offense is king in this metagame, and that probably won't be changing for a while with the re-introduction of Garchomp and potentially Kyurem-B.