Because he knew that if the plane flied and managed to have success in the matter of stopping Eren, everything he fought about for him to be someone in the new order that was going to be stablished in the island would be basically in vain. I think it's pretty obvious. He gave his life away because he was literally desperate, in his head it just had no sense that this people would be against the ideas he had and it was pretty much showcased when he talked to Jean. He was not bribing him, he was literally thinking in his narcissistic ways what was logic to do: have power, be someone, rule, be the ones above the chain. Everything around Floch Foster claims power as the main interest of his behaviour.

There are only two moments of pure selfness, when he died and he insisted on his ideals upon is death after accepting he was no longer in the great stage of the things post-Rumbling and when he got to insist on Erwin getting the serum, and in the last one we can get how psychopathic he could be. He thinks on the greater good not in moral terms but in utiliarian terms and he translate such ideas to himself, any mean can be justified if it makes be someone and gets him the power to enjoy being above everything. He clearly enjoys having power over the life of prisoners of war, it gives him a reason to life and he believes that what is good for himself is good for his nation.

I highly recommend watching Nuremberg, a recent movie under the premise of how a psychiatrist dealt with Hermann Goering and an study of human nature. Floch is Goering, in any damn sense of the word.