They're pacing things to milk the greatest number of resubs out of the beta process that they can.

First wave was people re-subbing for a chance at an invite. That happened mid-May when the closed beta began.

Second wave was people re-subbing for access to the June stress test. That happened mid-June, and probably pulled in a lot of folks who were not tempted enough by a chance of a beta invite but who were willing to re-up for a guarantee of three days. Also, coincidentally, it was right when people in the first wave would have lapsed out from bitterness after re-subbing for a month with no invite.

Third wave logically would be an "all access" beta in mid-July that will be opened up to all active subs from then through the start of early access. That will pull in everyone who was not tempted enough to re-sub for a chance of a beta invite or for a mere three days of stress testing. Again, conveniently, it will be right around the time that subs would lapse out for those who might have been captured by earlier waves. This also would give them a very accurate gauge of the population they can expect on launch day, so they can revise their server provisioning and load tuning up or down to accommodate.

Doing it this way guarantees them between one and three months of extra subscription time leading into Classic launch from pretty much everyone who is tuned in to Classic, which should make a nice bullet point for the next earnings call. Some have resisted the first two incentives for early re-subs, but it's unlikely many will survive the third and final temptation. And it will guarantee plenty of fresh asses in chairs for the final phase of whatever testing they want to do.

As others have mentioned below, it's clear that the early waves of beta folks are burnt out by now grinding on characters at Vanilla speed, all while thinking more each day about how their work will be wiped away in a few weeks. Blizzard probably anticipated this and smartly decided to limit early beta access tightly so that the big crowds would not be burnt out and long gone in the final beta month when large numbers likely will be much more important for effective testing.