Because bookmakers do not have unlimited funds themselves, and to stop them from losing too much money in one go that they can not cope with, they all have limits on both how much can be wagered on (or potential winnings from) an individual bet and also by one customer in one day. Because a run of losing bets using a Martingale system can soon require bets of 100's of times your original stake, if the bookmaker does not accept them, when you eventual win does come, it will not recoup all of the prior losses.