Because DVD media is very inconsistent in quality and through the past four years or so people experienced major problems and data loss with some kinds of media (alot of them in fact) , so , with so many media manufacturers , we here at cdfreaks and in other forums too avoid such problematic media .
For SL media :
TY , Verbatim and CMC are very good (Add to that my personal favourite OPTODISCR16)
For DL media :
Verbatim +R DL was the only brand worth buying for excellent quality and wide burner-support .
Sometimes other brands like CMC (as Arachne stated) can be rather good depending on batch quality and/or burner support but all in all you should not take the risk specially that DL media is rather expensive and if you got stuck with bad batch (most likely with other than Verbatim DL) you would loose alotta cash :bigsmile:
Verbatim in the other hand is very consistent in quality .
PS : I've NEVER tried DL media myself , but that is what I read here
EDIT : Arachne and Master Yoda were faster