I suspect that the developers thought short rests were going to be much more common than they are. The UA class design article highlights their thinking on the class:
"Warlocks have a unique spellcasting method, and they rely on being able to cast a smaller number of spells more frequently."
"The warlock spell list was carefully cultivated to avoid including spells that might become annoying if cast too often at the table."
The thing is, they couldn't be more wrong about how the warlock is in actual play (at least from my experience). Warlocks don't get to cast spells more often. It actually feels like the exact opposite. Warlocks cast their 2 spells, and then they have nothing else to do but use cantrips. With short rests taking 1 hour, I've rarely seen groups take more than one or two per day. So the reality is warlocks end up having maybe 4-6 total spells in an entire adventuring day throughout most of their career, and they have to rely entirely on cantrips the rest of the time. I really wish they had let us try out the warlock back during the public play test. If any class needed play testing, it was the warlock with its unique spell slot mechanic.