It's pretty simple.
People who play everyday and lost a ton of money on their site = not good because you are already spending all your money depoing and giving you a bonus won't make you depo more.
New customers and people who hasn't lost a ton of money = large source of untapped income for stake, give them a big bonus so that if they lose, you get the money back. If they win, eventually they will lose it all and redeposit.
Sucks but it's just the way it is, it's a business after all. Old players with no money are not worth giving bonuses to.