Primary turnout is not indicative of general election turnout.

[TwilightZone]

Never has been. Never will be. The myth that wouldn't die.

Before 2008, the record for Democratic turnout in the primaries was in 1988. Dukakis got hammered in November.

Democratic turnout in the 1992 primaries dropped significantly, by more than two million votes. Clinton won easily.

Article from 2016 explaining many of the reasons why there is zero correlation:

https://thehill.com/blogs/congress-blog/presidential-campaign/272381-the-truth-about-primary-voter-turnout/

[RandySF]

1. Trump brings white voters out from under the rocks whenever he's on the ballot.

That’s why polls underestimate his strength.

[yellowdogintexas]

2. Absolutely. We have even lower turnout for primaries

and runoffs are even worse. Low turnout in a runoff back in 2012 is how we got Cruz in the first place. His opponent was a regular Republican and would have been a quite satisfactory Senator. I think we had 14% turnout for that one.

Then we have the problem of open primaries, so both parties jump over to the other side to theoretically mess with the outcome. Anyone who did not vote in one party's primary can vote in a runoff for the other party.