I can offer a guess on that last question...
Symantec, Trend Micro, McAfee, etc. are large companies with the resources to pay Microsoft lots of money to get their products prominently mentioned on Microsoft's website. Since their products are all commercial (paid), they would not take kindly to having a free product appear in the same listing, stealing their business and costing them money. Microsoft is savvy enough to know better than to alienate these larger companies with deep pocketbooks for the sake of a smaller company offering a product for free download (however good it might be).
What you might consider would be to make a "Plus" version available, using the exact same software as your standard version except for a slightly different registration scheme and About box, charging US$29.95 for it, and then convincing Microsoft to list THAT on their site.
This way, the other AV vendors see just another commercial product available for purchase, and they feel less threatened. Microsoft gets the opportunity to list Avast without alienating their gravy train, so they're happy. And you get to keep offering one hell of a fine product to your user base for free while gaining lots of exposure (and maybe even a few sales at $30 each!), so you're happy. Everybody wins!