Question: Why is there so much emphasis in the Yeshivish world to learn Gemara in Aramaic? Why can't you find an English artscroll shas in most charedi yeshiva?

It is not Lashon Hakodesh and it was written in Aramaic as that was the language spoken at the time but nowadays where English is the vernacular why should Yeshiva students be pushed to learn Gemara in an extinct language.

Answer: 
According to Tiferes Yisroel the gemara was redacted multiple times by Ravina and Rav Ashi. The reformatted the words to make them more contemporary. The Masechtos they didn't get around to have uncommon words. But apparently even they didn't pull off a complete switcharoo.

Over those centuries Latin and other languages were interpolated. But no complete upheaval of the language.

Many times authorities are unsure of a word's meaning, and that word has halachic ramifications. We can't just get rid of the original and replace it with our possibly flawed translation. We must study it to become experts in it.

One example comes to mind is Rabbeinu Tam in Seffer Hayashar talking about business transactions on Chol Hamoed. He points out that Chazzal disallowed Prakmatia, a word only used to connote a business transaction, not personal acquisitions. This is a subtlety that might have been lost had someone previously translated that word into a more common, respectively modern, language.
It's helpful if said bochurim plan to learn almost any post-Talmudic Torah literature too. Commentaries on the Gemara, commentaries on Shulchan Aruch, responsa, and much else use a lot of Talmudic expressions.