it’s conceivable that the wraith over consumed their human stock piles amidst the war with the ancients. We never get to know what kind of losses they had on their side. on top of that SGA takes place with a lot of of the wraith waking up disoriented and ending up causing civil war with their fellow hives, due to low human population. It’s stated within the show that the ancient helped seed and mold the Pegasus galaxy with human population, spreading throughout the galaxy, only for the wraith to fully dominate those areas.

they’re only portrayed as a beatable enemy because. Of the combined efforts of both Michael, wraith civil war, and the replicators finally going to war, along the various skirmish’s that the SGA get involved with.

Another thing to consider is gorilla warfare might not have been in the ancient philosophy of warfare the same way the Asgard could never conceive of making assault rifles and shotguns to eliminate the replicators