People are fairly honest most of the time, but then they are tempted to cheat or take one small thing, or they see someone else do so. Over time this works through them, and maybe they take another small piece. After a while, this becomes habit, and people begin cheating at full throttle, and next thing you know, the whole bicycle is missing, figuratively. :)
The bike lasted as long as it did, particularly after the lock was removed.
We can rest a little easier knowing that people, for the most part, don’t cheat as much as they could, or as much as we would expect them too, rationally speaking—after all, just think of how many people walked by the apparently free, unlocked bike and ignored it.