Op first you need to understand, when the site give's registration bonuses to the users.
Normally registration bonuses are seen from new sites.
They do these in order to grab attention of the gamblers and this is the most common use of promoting a bitcoin casino or a gambling site.

And considering your case,
the site where you have registered might be an old and reputed site (like stake.com), for that reason there was no welcome bonuses.
Also everyone should note that if the bonuses are higher, then the stakes along with that might also be high.
So do a research carefully before investing a big amount in a specific site.