It's this. It's a business decision. You don't spin servers up in a second and take them down hours later, there's contracts involved. You spin up enough servers to handle the load you expect normally, not at launch.
Honestly I played Payday 1 A LOT, enough to be in the top 1% of 1% of players. Got invited to the studios after being among the first to complete the ARG.
Then played Payday 2 A LOT.
But I quit halfway through the lifetime of 2 because it was clearly not getting any better, but worse. They stopped innovating and just started looking at player builds and releasing more and more powerful bulldozers. Got boring really fast.
So when 3 was announced? I haven't even looked at it.