1. Only losers self-publish. It's an admission that your writing isn't good enough - and besides, you're using up your First Rights and no publisher will ever consider a book that's been self-published. (I've never found anyone to satisfactorily explain what first rights are.)
2. Self-publishing can be a good way to get the traditional deal you've always wanted - but you have to sell loads of copies on your own before a publisher will be interested.
3. Self-publishing is the best option: you'll make more money, retain your rights and have full control. Who wants a trad deal anyway, giving away your rights in perpetuity for a measly advance, no marketing, and a couple of months in the few remaining bookshops before your book is pulped?
Because these days, indie authors are the cool kids.