Actually you could play Master System games on the Genesis, you just needed an adapter, PS2 really kicked off the BC with the huge strong library of games from the PSOne, GCN on Wii, etc. SNES even letting you play GB games via adapter was really really awesome and gave more options.
Why is it a big deal? Because people want to sell their current console to make the new one affordable and when you lose out on the huge library of the passing gens, your left with a shitty launch library OR a list of games for $65 each, so it gets expensive and options are what make things more appealing. Losing this option makes it more difficult for someone to buy a new console and bare the brunt of that massive expense...and these new consoles could easily at least emulate the earlier gens (PSOne/2 or Xbox) but they don't...why? Probably because look at the PSN/XBL they're reselling digital versions of the games which IS neat for super rare games/playing on PSP/PSV on the go but why can't you just use your disc on the console? They want you to rebuy the game.
Why is it a big deal? Because people want to sell their current console to make the new one affordable and when you lose out on the huge library of the passing gens, your left with a shitty launch library OR a list of games for $65 each, so it gets expensive and options are what make things more appealing. Losing this option makes it more difficult for someone to buy a new console and bare the brunt of that massive expense...and these new consoles could easily at least emulate the earlier gens (PSOne/2 or Xbox) but they don't...why? Probably because look at the PSN/XBL they're reselling digital versions of the games which IS neat for super rare games/playing on PSP/PSV on the go but why can't you just use your disc on the console? They want you to rebuy the game.