'Desires better alliance' translates into 'Doing this does not further my cause'. In that if they agree they will not gain any dynastic claimants on foreign lands or things of that nature. Sure you would be a military ally, one that might refuse a call. They prefer to use marriage as a tool to gaining claims.
look we all know that that sort of reasoning makes sense on paper, but in practice, in the game, it's not that simple at all. half the time "desires better alliance" makes NO sense in ANY context... it IS an issue and apparently the Devs ARE addressing it.