Aren't Nexus devices getting their updates from Google? I haven't really paid attention since I was using stuff like CyanogenMod until I switched to iOS
Google apparently didn't had any contract in place for forcing TI to keep doing the updates in case of such situation would occur.
Neither they thought it was worth to spend some Google resources doing the updates themselves.
So they went to third option, letting their Nexus customers that they aren't any special and better buy a new Nexus.