They probably think the extra sales wouldn't be enough to justify a costly TV commercial. They are probably right, too.
I tend to agree with you, thinking in the short-term, based on how many sales they had of the Player's Handbook and my estimate of how much they make, on average, on the Player's Handbook. Taking 10% of that total for a marketing budget would not put them in the realm where they could afford a TV commercial.
But, what about more economical ways to use TV rather than just run a 30-second national spot? What about regional or local cable? What about sponsorships of programs that don't require a production budget, but just a static slide of the product (or product logo) and a 10-second "Buffy the Vampire Slayer is brought to you by Xander's favorite RPG - Dungeons & Dragons" thing?
My team had tons of other ideas for them, too, including stuff like a D&D Product Demo at the Premiers of the Harry Potter and Fellowship of the Ring movies. Imagine a team of WotC employees (or even the game designers) doing a small 10-city tour that did a demo of the game in the lobbies of theaters during the release of these two movies (the biggest fantasy movies of all time)? The cost of that would be a fraction of the cost to advertise the game on TV.