It hasn’t been able to thus far, same as every other “free” social media site other than Facebook. The problem is the sheer scale of the data that these sites have to main makes their overhead costs insane. To make profit from those ad spaces would require those ads to be too expensive to justify to the companies. It would be like trying to operate the New Orleans Superdome purely on ad space. The rate would be way too high to for companies advertising to ever get a return on their investment. Reddit can run ads to subsidize their costs, but it will never be viable as a primary revenue stream for them.