I see then.
Well the short answer that has been discussed countless times before is that the processor in the Roku won't handle it. Modern web pages are too flash and CPU intensive. The Roku was made with a simple CPU to run the OS and an awesome GPU to do video. It's the opposite of what you need for web browsing.
Well, I think some hedging is needed here.
The Roku's processor would not handle flash video, but could possibly handle other, less intensive flash content. That's irrelevant though since the processor is ARM based and there's no flash for it currently.
Theoretically the Roku could put in a simple component to display a web page, sans flash. HTML5 video and audio might actually be playable if encoded in a supported format. Control would be problematic, a remote is not the best way to navigate a webpage.
I don't think anything is coming anytime soon (and it would need to come from Roku, not a developer), but it's not impossible, just very unlikely.