Part-timers tend to be young, and they tend to be poor. If they move to chase lower rents, what are the odds they update their address with the union hall? They probably don't even know that they need to. Their address is on their paycheck, and their dues are on their pay check, so it's sorted, right? I'd guess that the share of part-timers who never even see their ballots because of this is stupidly high.
Is it the whole problem? No. We all know better. But a drive by the union to make sure addresses are up-to-date would be a good, easy first step to increasing participation.
Another good step would be to have the union promote that a vote is happening. We frequently get DRIVE cookouts in front of the hub, lists of endorsed local politicians, fight right to work petitions, ect. Can't be that hard to have a guy standing there saying: "vote in the IBT election", or "vote on the contract". I've never seen that. Maybe I just called in on those days...
The blame rests on both parties here. Part-timers need to care, and the union needs to make even a minimal amount of outreach. Simply shaming part-timers for not attending meetings isn't going to endear them to the cause. If you want them to take the union seriously, reciprocate that. Don't bury our contract proposal meeting at 8 am on a saturday like my local did.