In MG's case, I think it's pretty simple. If they could get better licenses, they would translate those instead of the horrid nukige. Without writing a wall of text about it, it's all about the licensing cost vs actual sales. If you take huge games from companies like Key, the music licenses alone would cost like hell, among other things (Kara no Shoujo voice license cost). On the other hand, a lot of VN companies are really xenophobic when it comes to licensing VNs in the west so they don't even want to talk to MG about it.
JAST - they most take already translated scripts and buy them from fan translators so they don't have many crap VNs.
Fan TL - Some translators TL some shitty nukige for practice or some really lame moege they like. The said VNs are mostly very easy so you don't need to know epic amounts of Japanese to TL them. A few rare translators tackle the more difficult and longer ones just for the hell of it. Personally, I think it's a damn miracle that some of these huge VNs got fan TL'd because it takes a fuckton of wok. Not to mention that they do it for free. We all complain about shitty VNs and slooooow TLs, etc., but there's nothing much most of us can do about it.