Presumably Madonna's songs don't appear because Warners won't allow them to be licensed to a rival company, and especially (oh the horror!) to a consortium of rival companies. It's a bit petty, but that's business.
Now 4 on CD goes for �250 ish on ebay the next one on CD was Now 8 which goes for �50 ish, if you ever see a Now CD under number 20 at a car boot buy it lol. A lot of "big artists" don't appear on the Now Collection probably due to their record companies. I know most of them are cra* but as a DJ they're a godsend!
'Tis true about Madonna never appearing. In fact, now that I think about it, it's one of those facts that I know, but I don't know why I know it or where I read it!
Madonna's first UK hit was in january 1984, so you would've thought that she would've been an ideal artist to appear on the compilation, but no.
Incidentally, when I wrote that last paragraph, I mistyped '1984' as '1884'.