Well, it's multiple things.

1. People are paid for their time on the council, and by law the pay is AT MINIMUM 25% more that whatever you were already making, but could be considerably more depending on the pool of eligible citizens. Remember, the pay is the GREATER of 125% of whatever you were already making in your private job, or 200% of the average pay for the eligibility pool. So if you're making $40,000 per year and get called to council, you're gonna get paid a minimum of $50,000 for your term, but if the average pay for your eligibility pool is $40,000, then you're gonna be paid $80,000 for your term. It's structured so that there's always a strong financial incentive to serve.

2. People don't vote because they feel their vote doesn't matter. When you're part of a pool of 10 million people, one vote is more or less negligible. But, when asked to serve, you're now one of only a handful of votes. Maybe one of 5. Maybe one of 11. But your vote absolutely matters in a way that nobody could dismiss.

And tbh, if somebody declines, it's really not that big a deal. Eligibility pools would be big enough that a nontrivial number of people could decline the position and we'd still have plenty of eligible citizens. Worst case scenario is come kind of coordinated general strike against serving on councils, but to be fair, if the population is pissed off enough to enact a general strike in a meaningful way like that, they would have enacted a Branch No-Confidence movement long ago.

It might be better to make participation mandatory to prevent a ruling class from demoralizing everyone else they way they did here with voting.