It's only the presidential elections where they would strategically benefit from maintaining the current, badly flawed system. It would actually directly and significantly benefit both parties to use score voting during the primaries, by greatly increasing the quality of their elected frontrunner candidates and therefore giving them a better shot at winning the presidential election. Even if they refuse to reform the presidential election as a way of keeping themselves in power (by refusing to overturn the system that gives them 2 party dominance), by using an improved voting method in the primaries they'd give themselves a very big advantage in the quality of candidates elected that would hopefully carry through to the presidential election, and we'd get improved presidential choices (i.e. no Donald Trumps).

For instance, if one party used score voting in their primaries while the other did not, there's a very good chance the party that reformed its primaries would immediately gain a large power advantage by electing truly popular candidates chosen without any of the vote splitting problems of plurality voting.

Now, they'd hopefully see the benefit for the country as a whole after using it in primaries and want to reform the presidential/senate/governor elections and so on to use the same, but there's no guarantee of that.

Pleas to both parties to at least use score voting in their primaries and the arguments why are detailed here for both Republicans and Democrats: https://rangevoting.org/ForReps.html / https://rangevoting.org/ForDems.html