Don't see why everyone is complaining and getting angry here? He asked a simple question out of curiosity.
But to answer your question simple profits. When there are fewer people on a server you're far more likely to dominant that server and for a premium member isn't that what you want instead of competing with 100+ other premium members. For this there is always going to be 1-2 people that will jump to a new server and dump money into it.
As for a merge; Not anytime soon, and if it is to happen it wont be for at least another year most likely and that if it's taken into consideration. To counter balance this they will have cross server PVP events.
The pros to being on a lowly populated server are shown clearly by videos like kaizer taking down the world boss, as a preium member when you take down the world boss being in the top 10 with 5%+ dm down you earn yourself 1,000,000+ Exp and around 15,000,0000 coins, where as here on Server 1 being the 1 dmg dealer at 1.8% yields us 350,000exp & 200-300,000 coins. Thats a drastic difference not to include your chances of obtaining items in GNW events and sage events has drastically increased. With this they have the ability to stockpile and grow at a much more faster rate of other players in populated servers and not needing to spend as much.(Unless you're dumping thousands into the game like some)
I personally enjoy a populated server, but to each there own and if they enjoy the small server then let them.