"new authoring standards" used.
All those new instruments and "new lighting / venue cues" are to blame also. All 'RB3 only' tracks won't appear to other Rock Bands to 'avoid accidents'.
Harmonix and EA have drawn a line under their original two Rock Band releases, with all future DLC too fancy for them to comprehend. The tracks are incompatible "due to the new authoring standards, inclusion of new instruments, new lighting / venue cues, etc."
All new songs before the cut-off date of October 26th remain in complete support of the first two Rock Band releases. You can't accidentally buy RB3 songs for earlier titles either as they won't even appear in any in-game store listing.
Certain tracks from RB 1 & 2 are eligible for Pro mode upgrades, at a cost, so they can be enjoyed in Rock Band 3 with its new Pro mode instruments. DLC for Rock Band 3 is much pricier now because of these new peripherals as it takes the studio more time.
EA had better hope Rock Band 3 sales pick up as both Harmonix latest and even Activision's Guitar Hero: Warriors of Rock haven't fared all that well since release.