In the early days they didn’t; that was the whole point of them. You paid a subscription specifically not to have ads like free broadcast television did.
It only lasted like a decade, but it was their whole selling point.
e: keep in mind, too, that broadcast tv at the time was where all the good content was. HBO only showed movies that had already been in theatres (thus the name Home Box Office) and Showtime’s hook was soft-core porn. (‘Do your parents have Showtime?’ was sleepover code for ‘can we watch kinda-porn after the ‘rents have gone to sleep?’) There wasn’t the dearth of original shows/movies we have now. They weren’t studios back then.
e2: sorry for multiple edits, but also bear in mind that when HBO first came out, people were watching their content on televisions like this, which was so inferior to movie theatres that ‘it’s in your home advertising free!’ was basically their whole selling point at first.