The original artist has moved on to work on other games and such. And supposedly you just can't "Ask him" to come to work on old shit anymore. Dunno why Mica doesn't just let someone else draw the main girls. If its art style, it shouldn't be hard to get someone that can draw in a similar or same art style as the 1st artist.
Well, I heard they got dumped by their original artist. That might explain why they aren't getting anything.
Not necessarily true, from what I know, Umineko left MICA due to conflicts in art direction, whether they left on good or bad terms, i dont really know and honestly i dont care to find out. AR-15 getting a costume is easy because well, Lin+ is the art director for GFL, so he can do whatever he wants for AR-15 (Even considering giving her a Kyuudo costume during that one company tour video).
But for the rest of Anti-Rain's case and SF M16, I imagine they were probably hindered on time and had a lot of conflictions on who would do their MOD3 designs, not like they could contact Umineko to do them, Infukun probably took on their designs since he most likely knows how umineko would draw them and keep them in line with the games theme without making them look out of place, he is good at replicating other styles.
As for why they dont have costumes, well... from my perspective... they dont really "need" other costumes, look at it like this costumes in the game are just fan-service and entirely optional. Perhaps MICA just doesn't feel the need to give them costumes, sure they have those orchestra dresses, but that was more of an exclusive thing to an external event (though honestly, i would love to see them in game, but i can dream, at least UMP40's dress is in through a furniture set bonus!), its not that is hard to find an artist to draw the costumes for the girls, its more of... they dont want to or dont need to. If they did do something for them... I'd imagine they'd make them obtainable via an event where everyone can easily obtain them and not something like a procurement pool.
Though... one thing did grab my attention a long while ago, out of all the characters Umineko's drawn (not just limited to GFL), RO635 is by far their most favourite design, if they ever eventually give RO635 a MOD3 at some point, i could see Umineko drawing it, but thats just a possibility.
Considering that they just got different artists to do QBZ-95 and 97, I think MICA is showing that they are now willing to let other artists adopt T-dolls now.