Because in "solid blue" places, dirty money knows a Republican can't win, so they throw insane levels of money into primaries to guarantee that regardless of which party wins, corporate interests never lose.
Like Bernie keeps saying:
We need to get dirty money out of Dem primaries
Dem politicians can fix our primaries on their own, because as the DNC loves to say, primaries are nonbinding and the party can do whatever the fuck they want.
Even if you don't think it's a problem because you think some of that money is legit, removing that money from primaries means it can be used in the general.
But as AIPAC keeps showing us, a couple million is more than enough to buy a primary race. Often it's price tag is only like 50-100k depending on the seat.