thats because people who want them, already have them... its not difficult to figure out cvg.
the consoles n question have been out for 5 + years, they are hardly going to keep on selling millions are they...
360 is down about 25% (but still sold about 1m), PS3 down 14% (but still sold around 850K), and Wii down nearly 60% (around 250K sold). sales generally plateau a year or two before next gen, so it's not a great shock.
what might be more of a concern is that PSP sold 400K in 2011, but Vita managed only around 300K for 2012. and Wii U launch sales have been less than Vita, and during the peak time of year game/console sales. the general economy isn't in the healthiest condition at the minute. games/consoles would be hit hard when disposable income falls.