because it would be a waste of time/money/space to use them?
seriously though, a media format can only really be classed as a sucess in the consumer market if it's actually used.
Minidisk was a partial sucess - it sold, but in limited quantities and was only really supported by Sony.
UMD is looking like a massive failure - it's getting less and less support from content companies other than Sony*, will almost certainly never be used on anything but the PSP (or it's sucessor). Even Sony seem to have a meh approach to the format.
Betamax - suceeded in the pro market where price/size is not a problem, but technical specs are vitally important. Failed miserably in the consumer market where it was expensive, bulky, and very poorly supported by content providers.
Re DDR, i very much doubt Sony invented it, as they are not a chip fabrication/design company and it requires a very specialised knowledge/skill/equipment base to design chips these days (hence why Sony went to another company for the cell).
I beleive it was one of the memory companies, who probably sold Sony the chips (or made some to their rough specification).
*I don't think amny companies have any plans to release more films on it, and many probably lost a lot of money.