It's a chicken and the egg problem.
Why won't people pay? Because the servers are empty.
Why are the servers empty? Because people won't pay.
Make sense now?
A lot of people won't pay, not because of the money, but because the premium servers in their region are dead. I see many people who have actually paid spending most of their time on the neutered public servers. There is even less incentive to buy pro than there is premium unless you've done a lot of networking and have a bunch of active friends. Not everyone wants to put this amount of effort in just to get into a good game of quake every once in awhile.
If you don't play duel then it's either europe or nothing really lol, because tdm and ctf are very much alive there (ca too obviously) but those 2 are dead anywhere else. Quake has always been euro centric after all.
It's not so hard to pay $8/m or ~$50/y and enjoy QL without any restrictions, limits etc. Too much words, too much "i want it for free" and other bla-bla-bla. In EU location i can always find CA/FFA/Duel active servers, it's not a problem.
Luv it? Support it, buy it
[You are a really good judgement of players behind walls © Seh]
I've had a pro account for a year, this is a month into another year.