Welcome bonuses becomes so popular that nealy all casinos use them. I visited different casino sites and neally all of them have a welcome bonus. It is necessary to conside welcome bonus like a marketing step.
Welcome bonuses have existed way before cryptocurrency casinos were created, a fast internet search seems to indicate the first online casino was opened on 1994 and as gambling online became more popular during that decade bonuses were needed to try to attract more customers, but bonuses are even older than that as psychical casinos have offered them for a long time as well, so giving away many different bonuses has become the standard and any casino which refuses to give bonuses to their gamblers will probably not remain in business for long.
All bonuses to encourage players came to cryptocurrency casinos with online casinos. Now the competition between casinos is so high that each casino, in order to increase its liquidity and increase the number of users, each time comes up with new bonuses or increases the size of existing bonuses in order to stand out from other casinos and show that it is better and more profitable for players.