The forum users do not vote for merit sources, reputable users apply to be merit sources and Theymos picks from the applications in the pipeline, when he feels there is a need to add new MS to the forum.
If I'm not wrong then the ones who applied for being accepted as merit sources need some type of support or you can say vote of the forum's members. If I'm not wrong then Theymos also considers that support as a positive aspect of someone's application and he's wise enough to know that the forum members opinion also matters when it's about betterment of the forum.
There is always need for merit sources but it takes time to know that who could be a better merit source and when Theymos is 100% sure that who would be good as a merit source then he could appoint that member as a merit source.
There are quite a lot of good users who have applied to be MS's and they have been waiting for a long now, but nothing doing from Theymos and i can't really say why; many of them deserve the "job" and they would greatly improve the merit system.
I have seen those applications and I have supported most of those applications because I also believe that the forum needs some new merit sources who may help out the members of the forum that need those merits. There are so many good posts by the members of the forum who are merit-worthy and those can get merits when new merit sources are added to the forum. If I'm not wrong then Theymos is also keeping eye on the applications and in near future he may announce the names of those members who get accepted as merit sources.