I don't really follow the UFC, so I don't really understand how popular Marlon Vera is but compared to McGregor people will be more aware of McGregor and it could be that his level of popularity makes Casino have to spend more to sign him become a brand ambassador, but it could be also that Roobet is targeting the Latin America market to be able to get a lot of players from there.
He is quite popular in UFC and headlined recently for the first time an UFC fight night (which he won). But he is only known by people who follow UFC a bit. McGregor is famous all over the world but to have a sponsorship with McGregor, you will need to have big bags of money
. Mainly in the future, Roobet signs other partnerships with some UFC stars (or other sport stars).