Brion-
Because it's MUCH MUCH MUCH MUCH MUCH faster than a longer cutoff, and it's fairly convenient to click now and again during the course of a day.
Rumor has it that there are actually people who use Wikipedia only for a limited time each day; in fact, I have heard (as of yet entirely unsubstantiated) claims that some people do not even use Wikipedia each and every day. Advances in Wikipedia technology will surely contribute to the assimilation of these unfortunates (recent Star Trek episodes highlighted some possible uses for nanotechnology in particular).
Alas, some of these wretched souls have ended up with rather large watchlists due to my "Watch articles by default" feature, so the 1 hour cutoff on large watchlists threatens their salvation. If we seek to make them part of our collective, we must accommodate their strange ways, for otherwise they may be so frustrated by the given behavior that they will consult our universe-encompassing compendium even less frequently.
Remember, until we can forcefully integrate all of humanity into the Wikipedia neverous systems, we have to come up with wacky reasons for people to join on their own accord.
Report prepared by Wiki-Drone #52