Probably a lot of the reason is less to do with the fps and more to do with the way one can employ such a weapon. By definition, a sniper requires a site which facilitates the role, ie. with a bit of open ground, or at an area at least not full of heavy foliage, although we're coming up on the time of year where that's less of an issue of course. Even so, unless there are obvious choke points a sniper can set up on, with him or her in a location which won't get rushed, which offers good cover/concealment and doesn't leave the sniper sat at the back with nothing to do unless they relocate when the battle moves forward, then it's not going to be much fun.
And even if all this is in place, it's still somewhat of a lonely role compared to being an important part of the support element for a squad. Basically, a DMR is going to allow you to play with team members instead of lone wolfing it (unless you can persuade someone to be a spotter with some decent firepower to hold the bad guys off, or to create a sniper team), which may appeal to some, but in reality there's just not likely to be as many opportunities for a shot in that role, as when pushing forward in a squad with a DMR in a support role similar to letting rip with a BAR or M60, but with increased accuracy.
After all, the notion of being a key member of your team is certainly an appealing one. As much as we would all doubtless like it to be the case, the fact is, airsoft weapons are limited in range when compared to their real-world counterparts, and there are never going to be the kind of situations a real sniper rifle could tackle with complete impunity, BBs just don't go that far, and even if they do, they are too affected by the wind to offer the kind of long-range accuracy a real sniper rifle brings to the battlefield, so a DMR is a more feasible option.
