What makes you think that ?
What exactly are you seeing that makes you say that.
I am running Yosemite and my TimeMachine backups go back to the year 2008 and I can restore files from that, no problem.
I had the same problem as the OP. I upgraded to Yosemite on my work machine, and then a week later a power surge scrambled my hard drive. I used the Restore function, and opted to restore from the most recent backup. Even though I had multiple backups made while running Yosemite, Time Machine would only install the last backup made under Mavericks. So I ended up with a Mavericks machine again, and had to re-install Yosemite. Concerned, I went home and tried to restore a file from Time Machine on my home machine. No go, I could only access files backup up on the previous OS X. Verifying the firewire backup drives fixed both problems. I have no idea why, but Time Machine now works.