Pretty sure it helped that Trump won the state by almost 20 points.
Another way to look at it is that Trump motivated hordes of anti-estalishment voters and Blue Dog Democrats who voted for him but also for Kander who would have otherwise stayed home. I honestly think that Blunt would have won by 6-8 points if someone like Kasich had been the GOP nominee. But yeah, his campaign was so terrible that I think he'll probably be gone in six years if Trump is reelected. Ugh.
This assumes that Missouri will even be winnable for a democrat in 6 years. Unless of course, you think someone is going to primary him.