My guess? Because even with a paid endpoint, the type of unscrupulous yahoo that is DDOSing IA today would probably still abuse the free endpoints because they can. The revenue that might come from a paid endpoint could help to scale up, but with how slow IA usually seems, I suspect there is an upper limit to how much traffic they can serve without a LOT more revenue.

This is a major "this is why we can't have nice things" situation in my opinion. IA is one of the most valuable gems of the Internet. The only thing that even comes close to preserving our shared history. The damage being caused (both by the effective DDOSing and by the knock-on impact that abuse has in encouraging publishers to remove their content from the archive) is incredibly serious.