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Timing
“This isn't a priority right now.”
Why they say it. It genuinely competes with everything else on their plate, and the cost of inaction is not yet visible enough to leapfrog other initiatives. It is a ranking problem, not a value problem.
Do not: Do not insist that it should be their priority, which is presumptuous and pushes them away.
Say this
Completely understand, you have a stack of things ahead of this and I am not going to pretend mine should jump the line for its own sake. What I would gently point out is that this tends to get more expensive the longer it sits, the cost just hides in everyday friction. Out of curiosity, what would have to happen for this to climb the list?
Then ask. What is ahead of this on the priority list, and is this quietly making any of those harder?