I haven't been in that many Chreidi yeshivot, but I gather that many of them speak Yiddish for the most part, not English. So, I don't quite see how Art Scroll would assist.
Regardless, it is always best to read something in its original text and try to learn the language. So, I would extend the premise of your question a bit further. Why aren't yeshivot teaching Aramaic so that students can understand the Gemarra in its original language? When I was in yeshiva, we learned Hebrew for about two years and spoke Hebrew to the teacher and our students in class for half the day. By the time I started learning Chumash with Rash"i, I barely needed the English translation to understand almost every word.
In 6th grade, when I began to learn Gemara, we had 5 pages of gilyonot - mimeographed sheets (who here recalls what a mimeograph was?) with Aramaic vocabulary words that we had to memorize and were quizzed on every few days. That was both before and coincidental to learning Gemarah. I should mention that Art Scroll didn't exist then, but Hertz and Soncino were around, but we didn't use them.
My point - inevitably, something is lost in translation. It's unavoidable. So, it makes sense to learn it in the original language. The problem, as I said, is that few yeshivot these days are teaching the original language. So, I think that much of your premise isn't completely factual.