Economics. Look at the money that the top European Leagues generate (Premier League, La Liga, Serie A, Bundesliga, etc.), not factoring in the Champions League. The Champions League/Europa League competitions alone generated 2.9bn (GBP) this past year. The Premier League alone generated 4.5bn (GBP) last year. The Bundesliga generated 2.7bn (GBP) this past year.
Comparatively speaking, Rugby League is a drop in the bucket (the NRL doesn't come close, the English Superleague barely makes money and is quite possibly too stupid to take money from the spectre of having a North American team earn promotion for 2018-19). Rugby Union doesn't pull in anywhere near these numbers (Super Rugby, Champions Cup, the domestic comps in Europe, the Six Nations, Rugby Championship etc.). Less money means salaries are smaller.
When rugby starts generating the billions that football does, they'll earn more.