I wouldn't say it's a drop in quality, just a drop in readers.

With a first issue, or a first issue of an arc or new creative team people will try the title out. If it isn't to their taste then they'll drop the book.
With the way pre-ordering works subscribers are tied to 2-3 months worth of issues so that's naturally when you'll see a drop, or if they've just given it a few issues to see if they will like it this will help retailers judge how many to order for the shelf for casual purchases.

I'm not a fan of the constant new #1's and trade-sized runs, but like you I do see it in a business sense.