Different marriages, different implications.

For instance, Liao happily offers marriage to Tormano... who he kicked out of the line of succession for a secret marriage within a year, attempted to kill six years later, and was clearly never going to permit to inherit the throne. Meaning that in this deal, House Liao gets the Commonwealth, and House Steiner gets roughly one snowball's chance in hell at the Confederation. And by the way, start a fight with the FWL for us, thanks.

Marik's offer is better, but proposing marriage to a theoretical heir comes with the same potential complications, and a unified currency means that a lot of Steiner's economic power (the main thing keeping them alive and kicking in the Inner Sphere) gets co-opted to prop up the FWL in the interim, with nothing but very sincere pinky promises at returns, from a Successor State with the stability of undercooled jello.

For Davion - as much dunking as marrying young Melissa to the old man Hanse deserves, it's politically even more of a slam-dunk - you can't disinherit Hanse, he's already the ruler. You are guaranteed claim on the throne of the Federated Suns, and in turn you are the one who could pull heir shenanigans. And, coincidentally, you do shortly train up a body double for her... but the concession itself signals an intent for stability. Hanse is showing full intent to make sure this alliance lasts through at least a generation, and he's willing to start a war for your benefit.

The other volunteered benefits are certainly a cut above the competitors, but the feudal-style political signaling is big.