The reason there is no backwards compatibility is, like pretty much every decision a massive company makes, motivated by money.
You don't make much by supporting games the customer has already bought for a system they already own.
You can say it's a "selling point", but in effect nobody buys the new console unless they intended to eventually play the new games on it anyway.
What's more, the manufacturer makes money from the games, not the console. Backwards compatibility actually encourages you to buy the console (perhaps even at a loss to the manufacturer) and then buy a minimal number of games for it, whereas you otherwise would've bought a whole bunch.
What's MORE, it actually encourages you to keep buying EVEN MORE last-generation, used games (which don't have the shitty DRM catastrophe) so that you buy EVEN LESS new games.
WHAT'S MORE, they have no justification for selling the old games back to you for a premium.
I'm an armchair fat cat.
Yeah but this logic doesn't work anymore in the age of digital goods. And the 360 was up to the neck into the digital age.
Most Xbox live users have a library of digital goods (games, indie games, avatar clothings, whatever), sometimes bought for a consequent amount of money.
Telling them that they have to throw it all away and buy new things because of a ridiculous hardware update is not really going to work for the console manufacturers.
Backward compatibility is becoming essential to the digital business.