I'm a new player, and I enjoy draft -- I went to my local card shop this week and draft didn't happen, though it always has before. The store owner explained it's because a new set is coming out next week - but I think I'm missing the point. Why does that affect if players want to draft, this week?
I believe it's because of how the store stocks itself, and because of the economics of a draft/prerelease.
Stores only have so much they can spend per week on their product. Given that they know they will have a lot of customers interested in the new set coming up, it would make sense to spend a large amount of budget on product for the upcoming set. (Or if they can't buy the product yet, save up for a larger spending the following week.) The store might have packs for sale right now they could use, but they probably want to save those to remain stocked on specific sets. They don't want to appear out of stock of certain things if they can help it!
Additionally, they want to make sure there's a large turnout when they do a draft. Most drafts will effectively sell packs for less at a prerelease, when you account for prizes (At my store, a pack usually costs $4. A prerelease gets you 6 packs and costs $25, and the store gives out an average of ~1-2 prize packs per person.) Thus, during a prerelease, each pack is effectively sold for ~$3.12. Also, the store probably has to have more employees present to accommodate many players during an event for handing out packs, enforcing rules, etc. A store might normally get by with just one or two people working, but an event might require 3-4 employees (or more, depending on store size). More employees means more money spent paying those people for their time.
To make it worth their while, a store will want to pack people in to sell a lot of packs. (If each item sells for less, you need to sell more items to make the same amount.) Many customers don't have the time or money to go to events every weekend, and are more likely to prefer going to an event with the new set that is hyped up then with an ordinary draft of an older set. The store doesn't want to put on the event that risks having only a mediocre turnout because everyone is waiting for next weekend.
Last thought: it's probably pretty tiring for the workers. A lot of events can stretch into late hours, and even if they don't it's still a lot of time to be working directly with customers. More people means more picking up and restocking afterwards, more cleaning the bathrooms, etc. A weekend off is nice, and helps make sure the employees are refreshed and ready to give their A-game next weekend at the prerelease.