Because the clubs (or whatever body runs the competition) are businesses, it is not in their interests to bolster international rugby.
The clubs have seen how much revenue the EPL generates and want an equivalent.
Private club systems are fundamentally at odds with international (Union) systems.
I certainly do not think that clubs are at odds with international rugby, the branding power of international rugby is second to none. Clubs will never sacrifice this marketing power but international rugby represent a core of at best 300 players across 10 teams.
It does not provide a sufficient amount of jobs and revenues to consolidate the pyramid. International rugby is the flashy icing on the cake, but only a strong club system can provide the cake core. You can't create an appeal for a rugby carreer with all the sacrifices it implies with a mere 300 discriminatory (per nationality) jobs on offer.
Even as a young rugby player in the SH, would you consider choosing rugby as a carreer path if the job market was only the few rugby super XV franchises with oldies grabbing their spots as if it was their lifeline.
Having an escape route toward European rugby provides a safety net and a constant renewal of players.
A (professional) sport can grow only if professional players can make a living out of it, international rugby is too scarce and risky to guarantee a living from rugby.
As soon as the choice of a professional rugby was made, creating a fully mature job market became the necessary condition of development.
It does not make life perfect but club system is the only way, you still need some regulators to guarantee fairness, competitivity, players welfare,.... And this is where the unions have to step in but they should stay out of all marketing and commercial aspects of the game.
My view, might not be shared by all but you can't have both a professional commercial sport and a union governed one, if you want the latest then the sport should have remained amateur.