The number of Renzokuken hits, your party's critical status, or the position of the moon and stars in the sky has absolutely no bearing on whether or not Lionheart is chosen as the finishing move. The actual ratios elude me at present, but if memory serves me right then the game will favour Blasting Zone over the other three moves. If anyone observes anything to the contrary, it is by pure chance, and a true long-term sampling of the finishing moves chosen will eventually reveal the true probability.

And before anyone asks because I know it will come up, no, upgrading to the Lionheart sword straight from Revolver will NOT result in you only having Rough Divide and Lionheart. When you upgrade your weapon you will gain all of the limits available to the weapons below that level, so in the case of Lionheart you will have Rough Divide, Fated Circle, Blasting Zone and Lionheart.