The old rule was congress critters weighted a personal letter with highest weight over a form letter, over a call, over an email. Now, staffers are of younger age, and they are the gate keepers of what gets passed along. So that order of weighting may be different now. Then again, that's probably also highly dependent on the specific congress critter. AOC may be much more open to tweets/emails/etc vs Mitch McConnell or similar aged someone
They might have tried attaching this to Artemis, as you suggest, but gotten political pushback. They might have known that they can get Artemis approved without the DSN improvement and then hold Artemis hostage for DSN improvements. DSN money might have ended in the wrong Congress person's district. So many possible political reasons we might never know
Perhaps, but that's largely not in NASA's control; Congress appropriates money for specific purposes. NASA can't legally go grab Artemis money and shift it over to DSN.