That's simple because, there isn't enough quality posts been created. You don't expect meriters to hand out merits to undeserving posts (in their opinion) just because we want more merits in circulation.
That is exactly what theymos wants Merit sources to do:
It's best if sources try to exhaust their source allocations, even if it means giving posts higher amounts than is typical. If you have 150 source merit and you only see 3 merit-worthy posts in a month, then I'd rather you over-give each of them 50 merit than let the merit expire. That way there are more people capable of sending merit, and the "merit economy" is less top-down.