Doing M/F with 5x5 straight sets is probably a lot more than most can take coming to this from a BBing background. Looking at results, most don't seem to need that kind of workload although a few do and several here have been progressing to the point where it will likely start showing up sooner or later. With the lighter workload it also makes the program more accessible. If someone comes to it who really isn't ready, it's not enough to really break them in half and they still see solid progress.
Of course the other thing is that with setting this up to equal current PRs on week 3, and to be conservative, you really don't wind up with heavy loading. Maybe towards the end you get fatigued but for most, they likely aren't bombing away for 14 days straight (which is a lot for anyone to load). So this is conservative, let's a greater number and broader distribution of people (skewed toward the novice/intermediate) get out of the program with good results, and allows for people to push a lot harder later once they know what's going on. Basically, better to get 60-80% than 0% and blow up - and most people are typically used to 10-15% if they are making progress anyway, so it comes out in the wash. In reality, setting PRs is going to be very hard (maybe impossible) if you are overreaching and have good estimates - it's the trying that matters.
Glenn kind of has the advantage of knowing the people that use his programs and adjusting them accordingly, and his credibility is golden. Plus, people who know who he is generally aren't the BBer types who know very little about training. When I first posted this here, I knew I had to make it accessible (although I never thought real novices or beginners would be trying it) to a broader audience and get almost everyone results. Because if a few people bomb - it's the program that sucks not the 'man-geek' who tried this after 3 months of Bowflex. To get people interested in real training, I had to show them something they had never seen and could easily confirm (i.e. dual factor theory) and then provide a program that was easy to understand, accessible, and got big results. So in the end, I made it conservative and later gave them the tools and knobs to turn to make it harder (i.e. PRs in both 3/4 and 8/9 rather than just a new PR in 4/9) and if people found they required additional volume to step up the 1x5/1x3 days. On the whole though it's worked fairly well though. Probably about right for most people in this forum before the program got popular and started attracting younger and less experienced lifters.
EDIT: oh yeah, as anotherbutter's said below, if you are making really solid gains in the first phase - this has nothing to do with periodization, this is all about proper structure in a program (frequency, volume, intensity %1RM). If you are shooting the lights out, you'd be better served with lower overall workload and more consistent PRs (i.e. why wait once every 4-8 weeks when you can ramp for 3-4 weeks and then string together 6-8 weeks of them - it's a lot more efficient use of time). Of course, telling that to people has to be done nicely as it isn't this "new dual factor theory" thing but a "you don't know anything about program design thing". Kind of important to remember that we all started ignorant and those who learned the most just kept at it and learned from a lot of mistakes.