Read the terms of service and conditions before playing with those sign up bonuses. Most of them have almost impossible to achieve rollover requirements, while the rest will have a limit on how much you can withdraw from your bonus wins. Some even won't allow you to play certain games, and if you do, you will lose all your bonus/winnings when trying to withdraw your funds.
What could be the possible caused of the companies that are not giving the sign up Bonuses?
People always finds a way to abuse such promotion. Hence they don't like giving sign up bonuses.