This matter has been debated on meta previously. It seems that some merit sources are not active, some others find it difficult to find (quality) merits to post but what I hadnt seen commented on here is this:
I guess when the system was first implemented those who werent Merit Sources had more sMerit & naturally would be more likely to hand it out. Its been a while now though & lots of posters may simply have run out.
That might be one reason. Everybody with certain ranks was given merits to send initially. Another one is that we need more (active) merit sources. On that, most people agree.
When I have merits to send I send them pretty quickly because I have a broad conception of what a quality post is. I don't send merits to shitposters, though. Others dont send them so easily because their concept of what a quality post is is narrower.