Wow, you're old. Never knew that.
What's the reason Glenn crashed?
<shakes fist> GRRRRR. 40 is
not old. Particularly when you look 30ish and have a 16 month old son. :-P
Glenn had a ton of hype based on his impressive biography as a space hero, which was augmented by the big Hollywood blockbuster "The Right Stuff" that came out at the time and was very flattering towards Glenn. Plus he was seen as the relative "moderate" in the primaries that pundits thought might be a good Democratic alternative in the general election against Reagan, and the primary race was accordingly at first seen as a Mondale-Glenn race.
The problem is there was relatively little to back the hype. Glenn certainly wasn't a bad senator, but by 1984 at least he hadn't accomplished anything nationally noteworthy. He was decidedly charisma challenged both as a speaker and in general. And he ran on little else other than his patriotic biography and general "moderate" Democrat image (even though he and Mondale agreed on almost every major issue).
Suffice to say he failed to ignite much interest among the party faithful, and placed a decidedly poor 5th place 4% showing in the Iowa Caucuses, which mortally wounded any serious chance he had at winning. A week later he finished a weak 3rd place in NH which Gary Hart won, turning the primary race into a Mondale-Hart match up in the media. Glenn limped on until the first ever Super Tuesday where he peaked with a 3 way tie for 2nd place (far behind the winner, Mondale) in Alabama then dropped out a day or two later.
In a nutshell, he lacked either the charisma, record, or ability to articulate a compelling vision for America to match his patriotic biography and initial media hype. Thus I doubt he would've done any better against Reagan should've luck and circumstances somehow made him the nominee.