First of all, it's not necessary. Second, the company is doing some tests if there will be lots of sign-ups just by offering other promotions and not a signup bonus. Sometimes, it does not even matter as long as the other promotions are good to our eyes and worth our money.
The budget also plays an important role. Instead of a sign-up bonus, why not just use it on marketing or promoting the site. If their bonus offer is good, users will sign-up on the site even there's no signup bonus.
But the reality is, every site should at least have that kind of bonus as a brand new site.