You can only install the OS (and up) that came with the computer. It been like that for many moons. It may have been like that since the Mac came out. It all has to do with the hardware (chips). I'm sure a lot of Window computers are like this too. New hardware new code has to be written for it... new OS. If you go backwards there's no code in the OS for the new hardware to use...
I just ran across this post today. You may not get my recomondtion but it worked simple and great for me. I have FCS2 and of course it won't install on new iMac. What I did was leave installed on old Mac and simply isolutated the application DVDSP4 and burned it to disc. I then installed DVDSP4 alone on my new iMac running OS 10.7.5 and it installed like a breaze. I then inserted the media discs for DVDSP4 and the installed too. It's amazing. It runs better on this Mac than it ever did on my G5. Absolutely no hickups, crashes or anything. Perfect All you need is a Mac old enough to intall FCPS2 on and you are set.