headfairy yes, of course the money and bigger readership is a factor, but so is the frustration with the tabloid lack of ethics and fascination with gossip and dirt.
Many of the writers/journalists I know do their jobs because they have a genuine pride in producing good work, and especially with investigative journalism - getting to the bottom of major corruption/a story and pushing it to the top of the agenda, so change can happen.
I don't think the same can ever be said of the red tops. Sure, they break a story on a sleb playing away from home, or an MP that's gay, but in the grand scheme of things, that kind of news is bollox and unimportant.
I'd far rather read about the scandals of MPs expenses (telegraph), Iraq War dossiers (Independent) or the downfall of Murdoch for corruption (the guardian).