There was plenty of hype around the phone prior to its release as evidenced by the fact that it sold out so quickly. There was no reason to push too hard in marketing it further because the big competition was still a little ways off. Enter the Droid X and iPhone 4. There is hype around those as well, so then it is time to lay on the advertising. Keep the Evo hype high and try to steal a little thunder from the other guys. So now you have more demand than you do supply, leaving you with two choices. You let the hype die down (e.g. stop advertising) just because your supply is low and all the would-be customers lose some of their excitement for the Evo and buy something else. Or, you keep them excited about the Evo and convince them to wait it out until they can get their hands on one. If you go with the first option, you lose those customers for good. You don't steal away a customer 1 month after they signed a 2 year contract. Option two gives you a chance to capture them, still.
As for investing in production, it isn't like playing Sim City. You can't just take some money out of your advertising budget and make phones appear. If it were as simple as throwing money at it, don't you think they would have done so already?
:facepalm: