Because as a self-published author, either you are limited to e-books...or you have to pay a lot of money up front - on editing and professional-quality cover art - to produce an attractive print version....It seems, however, that these and other commenters who mentioned cost are using old information. Or they haven't conducted complete research and discovered the full range of self-publishing options available. Not so long ago self-publishing involved paying a hefty set-up fee, then having to purchase a large number of books you would sell yourself. Stories of boxes of books in writers' garages are legendary. The initial investment could easily be two to three thousand dollars.
One, because I lack the ability to do two things at once (ie: write a novel AND market it). If I self-pubbed, I'd need the resources to hire someone to do that part for me, and I am too broke to do that currently.
...I chose to persue the traditional route for a couple reasons. I don't possess the resources required up front for a first class publication....
Today, however, while that arrangement is still available, two other options to self-publish are very reasonable. Electronic self-publishing (eSP) involves zero upfront cost, unless the author needs to hire out formatting and covers. Well, a freelance editor may also be needed to make the text book perfect. Still, the cost of covers and formatting are very reasonable for a full length book. Freelance editing could be expensive, I suppose, though I haven't looked into the cost of that. Options such as critique group and exchange of beta reader time and effort are ways to offset those costs. But for the writer who can format and edit sufficiently, and if you accept just a slightly lower quality of cover cost (presuming you can't do it yourself), the upfront cost to eSP is quite minimal. And there's no initial inventory of books.
eSP, of course does not put a physical book in anyones hand; thus this may not be fully satisfying. For physical books, the inexpensive alternative is POD—print on demand. This relatively new technology has improved by leaps and bounds the last few years. Cost of the equipment has come down, and the quality of the bound book has improved. People who have bought these (I haven't yet; haven't been any place that had one of the machines, nor ordered any books that came that way) say you can't tell the difference between an offset print book and a POD book. Offset printing costs more for a small print run than does a single POD book, but a POD book costs more than an offset print book in a large print run.
So the negatives about the cost for the author to self-publish as compared to the traditional publishing route have pretty much vanished. Part of the reason for this is the cost to the author to traditionally publish. Yes, there are costs involved, and I don't mean lower royalties. I mean up-front costs. First, the best way to break in to trad-pub is to attend conferences, meet agents and editors, attend classes, network with anyone and everyone you can, and pitch your book at each opportunity. Those conferences cost money. With travel and tuition it could be $1,000 per conference, and you might have to do that for years before you attend the right conference with the right agent or editor having the right product to pitch. Thousands of dollars.
Of course, you could say that you need to attend conferences for the classes and networking, even if you don't pitch a book, just to grow as a writer. I won't argue that point, other than to say conference attendance would be a whole lot less if you eliminated the chance to meet editors and agents. I don't think that many people would go simply for the classes and the networking.
Then there's the cost of a freelance editor. Yes, read carefully all the advice given by publishing professionals (agents, editors, publishers, already published authors), and you will see they all recommend that you hire an editor to edit the manuscript you intend to submit to a traditional publisher. When signing a first time author, publishers want a manuscript that doesn't need a lot of editing. That's what all the experts say. So really, there's not cost savings there. If you need an editor to self-publish, you need an editor to pre-edit your work before submitting for traditional publishing. The cost is the same.
Then there's the cost of time and emotions. Once the quality of your writing is where it needs to be for acceptance by a traditional publisher, that doesn't mean you will be successful at getting it placed. Publishers are the buyers in a buyers' market. They turn down excellent books all the time, making a judgment of what might sell by the time they can get the book to market. Or they may have just contracted for another book the same as yours, and don't want to have two competing books. Or any of another hundred reasons why they may have to pass on your book that is just as good as others being published, and maybe better.
And finally there's the emotional cost of dealing with rejection after rejection, of waiting, and of wondering if you'll ever break in. That will be more of a concern for some than for others. Rejections strike different people in different ways. We all know it's part of publishing, and so we become philosophical about it. Still, there's an emotional cost for everyone as they process the rejections and the wait times. Those costs can be beneficial, but they are still costs.
So, all in all, it seems to me the cost to traditionally publish is not less than the cost to self-publish, and may in fact be more.