There's two main reasons why SS and SL prescribe only 1 working set of deadlifts. First is that the 5x5 squats 3x/wk and the 5x5 rows every other workout are plenty of volume for the musculature involved in the deadlift, and so you don't need much volume on the actual deadlift to progress on it. Second is that with all that squatting volume throughout the week, you've already got a lot to recover from, so if you can get away with less volume on deadlifts and still make progress, Rip and Mehdi would both argue that you probably should, so as not to overtax the body. Of course, that's their thinking, and it has its place but certainly isn't absolute truth. There's always alternative approaches, such as Nick Horton's mantra that "more isn't always better...but it usually is."
Outside of SS and SL, on a program where you aren't squatting and deadlifting on the same day, you're more likely to see a greater volume on the deadlifts. I've seen plenty of bodybuilding, powerlifting and general strength programs that have the exact same amount of volume for both squats and deadlifts, but they tend to be done on different days of the week.