Ask yourself what kind of markup we'd have to charge on storage space and still be able to grow our business when most of the repos we host are less than 1 MB.
We charge what we do because it makes money. Money that allows us to continue hiring really talented people that are all focused on building an even better service.
Doing things like including private repos with our free plan would eat into our margins and only satisfy the people that are likely to never convert to a paid plan. Frankly, I think being able to use all of the tools we provide for the price of a pint of Guinness every month is a damn good deal.