Indeed. They became well known here because of their different marketing strategies (sig campaign, contests, games etc.) the reason why they became popular. Though they already lessen their promotions as well as their sig participants.
Many constructive factors for their success.
They must build up good products (games) and partner with well-known game providers.
They must have good marketing with things you mentioned (signature campaigns, bonus, promotion, contests, etc.) and loyalty programs to build up stronger connections between their business and users.
They must be transparent, fair in processing users' withdrawal requests include money from won bets.
Together, those factors help Roobet to build up a good business, good reputation and keep it sustainably running with time.