I fear that as soon as something becomes this controlled by politics, reasonable project decisions take the backseat to political concerns, no matter what any participant's motivations and reasoning is.

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