I'm guessing the only thing stopping tomato from being developed on other platforms (like atheros) is someone having the time and the know-how. Up until fairly recently, we didn't even have 5GHZ support until Teddy_B stepped in thankfully to help with that development. Unfortunately it looks like Teddy_B is taking possibly a permanent sabbatical from TomatoUSB. He really has set things up pretty well, and the remaining developers have been able to pick up pretty much from where he left off.
From what I understand, however, it would be quite the feat to integrate something like atheros in Tomato. So many systems rely on the drivers, like wireless, switch, nvram. I couldn't even begin to fathom what kind of involvement it would take. Broadcom just happened to be the device the original developer of tomato (Jonathon Zarate) was developing for. The developers, at least so far, simply don't have the time or the compensation to make it worth while.
The creators of DD-WRT have created the time and compensation for themselves by commercializing their product. I see this as a two-edged sword. First, this has angered many people, because dd-wrt started out as an open source project, they forked the code, commercialized it, and have made it difficult or impossible for me or you to compile it. They still make the firmware largely free, but charge for *special editions*, and will also create custom builds for a price.
Secondly, however, since it has now turned into a business for them, this has allowed them the time to support so many devices. It is now money that motivates their development, to make their services as widely available as possible. Without becoming commercialized, I highly doubt DD-WRT would be where it is today, and support as many devices as they do. Even thinking about it now, you want to be upset for them taking things commercial, but they've been able to take it to a level of availability it otherwise never would have seen. I feel, however, they are spreading themselves too thin over so many devices. They're the jack of all trades, but master of none.
Going commercial however, would most likely involve going closed source, which doesn't really help community involvement, and also goes against the open source licensing. There is one branch of tomato called Dual Wan tomato, that forked from this project. They really have developed some nice technology, which would be nice for this project to have, but they've decided to close and commercialize their source. It's against the licensing, but not really anything that can be done. Open source is largely working on the honour system.
If you do like the development that tomato has continued, I would highly suggest donating to the developers. They mostly incorporate eachothers work, so don't forget to maybe donate to a couple.
Personally I'm fine seeing TomatoUSB stay broadcom only, though I would definately welcome additional support. With devices like the RT-N16, E4200, RT-N66U, these are some respectable routers, and I know for myself, and probably for many, I will only purchase devices that are supported. Yes it's less likely that someone who stumbles upon tomato will have a supprted device, but that just is the circumstances of open source like previously mentioned.
Sorry for being so long winded. That's my take.
I was speaking largely in general for other people too that have had this same question.
edit: I would absolutely love to see you or someone branch out into atheros chipsets. That would be just the edge tomato needs to get wider recognition and a wider audience. I'm not a C programer, and really don't have a grasp of what it would take, but I'm sure many would appreciate your efforts.