Because of the difference on bankroll. Rich people have a huge bankroll that gives them freedom to increase bet or play more until they win huge amount while poor has only limited bankroll which they will stop immediately once they lose enough.
That's a valid point, the size of your bankroll has a huge part to play in contributing to he success of a gambler but on the other hand has its disadvantages too. Having a high bankroll also means you have more risk to manage, alongside having a huge and high bankroll you'll also need to have a good funds and risk management skill, else you'll loose more than you could've ever imagined.