Fancy the price tag on OD&D (in hard copy) ? Well, lots of people wouldn't, anyway. Just one example that springs to mind.
Sometimes, they're quite convenient, even when finding and/or affording "the real thing" isn't an issue.
Some of this could work well for WotC, as well as for many gamers. What are people who are interested in playing OD&D, but who can't or won't afford to pick up a copy, going to do at the moment? Ah yes - quite possibly, anyhow - buy Swords & Wizardry (or another close-ish alternative) instead. Money that could've gone to WotC, but instead goes to whoever happens to be selling the "retro-clone" in question.
And if you look around, you'll see that *almost every single other* RPG company on the freaking planet does sell PDFs. Oh yeah, and WotC used to have all those D&D PDFs up for sale too. . . totally doable, just not, y'know, being done.
That aside, in general, what with the proliferation of iPads and the like, PDFs are only becoming more and more serious contenders for gamers' backpacks, and er, "shelves". IMO, anyway.