I agree with Beeftaco. I'm kind of shocked that Solrock and Lunatone never really received evos to this day. It seems like they've been forgotten about, or something. .___.;
My take on it, if they haven't been given an Evo, then they a) Aren't supposed to get one...like ever. There are some that are made the way they are on purpose. They are more for story than battle and add a variety of gimmick and novelty points for having.
b) they don't think its needed yet. They have a couple of binders of un-used designs that they can pull out every new generation. If they feel that they want to evolve a certain Pokemon and had something already in mind then they may pop up then. If they don't use the designs for new Pokemon like Tirtouga and Munna from Gen 1 Beta.
As much as I would like a Solrock/Lunatone evolution I can sorta understand now why they haven't got one, they're supposed to be stand alone Pokémon that are counterparts to one another so it wouldn't really be right to give them an evolution. It's the same with the likes of Plusle and Minun, Volbeat and Illumise etc.