What could be the possible caused of the companies that are not giving the sign up Bonuses?
•Whether they have forgotten. 
•Whether they dont have budget at all.
I doubt they have forgotten. A lot of people will make sure to remind them of their bonuses. So it might be bad budgeting from the side of the companies. And even if you do get these bonuses, a lot of the ones that look too good to be true are actually bonuses with hooks attached. So for example, you get a large bonus for something, next step is you need to play some game or do some thing before you are allowed to withdraw it. Ive seen it before, but only in shady gambling websites.
Most of the time gambling sites know how to inform and plan.
I guess this is not one of the trends because in the early start of the gambling platforms this is one of the common marketing and also the gambling platforms never get less budget unless only a few players, also the Rakeback or bonuses coming up with the gamblers so they feel the platform really care to them and would like to play more, some of the trend regarding with bonuses is the Rakeback and largest better in the leaderboard.