I think hester makes a good point in saying that the voting system often leads to maps that most players find "okay" but isn't necessarily their favourite choice.
In other words, the maps like Radar, GRush, Delivery, Supply, Beach ... are not necesserily played because everyone loves them but because everyone can live with them ...
However I don't see a good alternative to be honest;
as Cuckun said there is already a barrier and I think it the worst thing one could do is to make maps not playable for an even longer time (longer than 10 maps) or even force certain maps (jaymod-like rotation), because in the end that will lead to even more complaints about certain maps. I think the majority is quite okay the way it is ...