It's a plethora of issues that combine into one hideous, mind melting beast that makes backwards compatibility less enticing for big businesses.
On the PS3, they used to include an extra processor so you could run PS2 games. They since stopped, partly because it was cheaper, but also because the PS3 had the new digital store come out where you could buy all the old PS2 games digitally. The downloadable PS2 games would be optimized for the new system, unlike the disc version. Boom, Sony saves money and makes money. Expect to see PS3 games on sale in the PS4 digital marketplace some time in the future.
I imagine the same will be true for the Xbone.
Games are also running off of a completely different OS in new systems. It's like trying to run the Windows version of a program on a Mac, except the system is designed to deny any game it doesn't recognize as "Xbone Disc" or "PS4 Disc" as part of its DRM.
Another factor is that the hardware architecture itself changes, and these games are so optimized towards a certain build that you never know what problems are going to come up. If you're lucky the game might run, but there's no guarantee, and a butt load of testing would have to be done for hundreds of games to ensure that it works across the board. It's like trying to run a DOS game on Windows 8, except the DOS game was so optimized towards a particular DOS computer that it screws up in places you wouldn't expect on the new OS.
The easiest way to fix that is to include a different, older processor and GPU, but that's more expensive. You could do what the Wii did and include an emulator, but that requires disk space and man hours without a guarantee of success across the board. You're essentially spending money on something that will, at most, earn you a few kudos, but kudos don't generally earn you millions of dollars.
The easiest, cheapest thing to do is port previous generation games over later as an HD remake or "download only" version that's been ported over to the new hardware. Notice that Microsoft also introduced a smaller, more expensive 360. You can bet that they'll cycle out the old, cheaper 360 shortly after its release. There's some more money to be had from the folks willing to spend it so their $2,000 gaming collections doesn't become worthless.
It is possible to make a system that's fully backwards compatible. It's just not profitable.