As of time of writing, there are over 300 inactive/stale challenges on the map as reported by FOAM Daily Digest, waiting for someone to spend gas to Update them. Over time this number grows slowly, but steadily. This gives wrong impression of the actual challenge activity level on the map.
Primary reason is probably insufficient incentives: prospective gains may not cover the gas cost of Update transaction. Secondary reason might be UX: challenger and voters forgetting which challenges they got involved in. Also, there are no protocol incentives for curators outside of the challenge to Update.
To provide an incentive, FOAM protocol could be updated in the following way: when a challenge is issued, some additional amount of challenger’s tokens is locked as Update Bounty. The Bounty is awarded as part of the final Update transaction. If Update is executed on the first day after the challenge ends, it is awarded to the challenger; this motivates challenger to Update and avoids gas racing with others. Otherwise it is awarded to the account executing the Update; this motivates curators to find out and Update expired challenges.
This still leaves an issue of existing 300 stale challenges, but would keep their growth in check.