Well, first of all, that's not strictly true. I've recently joined the Drupal community, but I really like Rails and wouldn't hesitate to recommend it to the right person.
But, more importantly, when a member of the Drupal community inevitably urges you to use Drupal, they might also give you specific bits of invaluable advice like "oh, that thing you're trying to do is just Drupal core, plus CCK, plus Views, plus Organic Groups, plus Imagefield/Imagecache. Oops, except that Imagecache assumes X and you want Y... you'll need a bit of custom code. This other thing you want to do is 100% custom code. And those fine-grained permissions you want will add an extra 100 queries per page for logged-in users -- they'll stop working as your paid userbase grows, so remember to hire a hacker to re-engineer them if your app is a success. And make sure to budget extra time for theming, because it's a bear."
But, more importantly, when a member of the Drupal community inevitably urges you to use Drupal, they might also give you specific bits of invaluable advice like "oh, that thing you're trying to do is just Drupal core, plus CCK, plus Views, plus Organic Groups, plus Imagefield/Imagecache. Oops, except that Imagecache assumes X and you want Y... you'll need a bit of custom code. This other thing you want to do is 100% custom code. And those fine-grained permissions you want will add an extra 100 queries per page for logged-in users -- they'll stop working as your paid userbase grows, so remember to hire a hacker to re-engineer them if your app is a success. And make sure to budget extra time for theming, because it's a bear."