I don't see much sense in voting on queries, either up or down.
Upvotes there are pretty much replaced by favorites: if one think a query is good he'll favorite it and unlike all other Stack Exchange sites, there is list of queries ordered by the amount of users who favorited.
As for bad queries you have a point there, but since nobody can delete queries (maybe a dev) better just leave them alone.
If you want only good queries, just browse this list.
As for the "why" itself, Data Explorer is a standalone site which is not part of Stack Exchange and not using its "engine", just its data dump. So the whole voting system was not implemented by the person who created DE (Sam a.k.a waffles, former SE dev), and adding such thing is LOTS of work.