Why didn't they go with a "break the bank" end game common in 18xx?
Because we weren't looking to design an 18xx game. In particular, when the endgame is triggered, due to the different rates companies operate, the game can go on for a variable number of additional rounds. We felt having an end-game trigger was a cleaner solution to this problem.
That said, I would encourage anyone to play the game the way they enjoy it. We never playtested a fixed bank, but my suspicion is that $75 * the number of players ($150 with 2 players, $225 with 3, $300 with 4) should be about right, unless you'd prefer to extend or shorten the endgame.