I think it's a question of cost.
For instance you know how Anti-viruses sometime come pre-installed on PC's. Well PC manufacturers don't do this for nothing. They are paid by the Anti-virus companies to install demo/trial versions of their product.
Similarly, creating a package and selling it to a chain store which in turn sells it to a customer costs money.
I would guess, it costs more than having a server from which people can download your product.
By these arguments I believe (this is my opinion... not fact) that Avast will not be available to be bought in-stores.