The crucial bit is this:

'“Staying out of primaries sounds small-D democratic, very intellectual, and very interesting,” said Hoyer. “But if you stay out of primaries, and somebody wins in the primary who can’t possibly win in the general,” the Maryland representative said, citing the surprise victory of Democrat Doug Jones over Republican Roy Moore in the Alabama Senate election, “I’m not saying you’re that person.” But staying out of primaries, he argued, is “not very smart strategy.”'

And he's right. It's not "slimey", it's how politics works. The national parties have a strategy to win elections. They put their bets on what look like the best horses. That's politics.