The planet will have obvious inhabitability. Europa has killer radiation within one earth day and we're flocking there. Give the kerbals ability to breathe but make other planets the better choice. Also, Laythe. People may go there for Jool because it's cool for being habitable, but they still go to other worlds! Tylo's a massive destination, as is Vall, Bop and Pol. Have the option available for those who want it. But others can gravitate to the world's that have other good things about them, like being resource rich. A good 90 percent of new star systems get you a new habitable world to toy with. The Blalo issue is real, but one habitable planet (or moon, habitable moons are great) per system is reasonable. Just one though.
(P.S. Planets can be tidally locked on other stars, or non-active red dwarfs.