Having read that book til it literally fell apart in my hands...
I think there are a few factors at work. Ampeg was usually seen as the "East Coast Fender." And, later in the paragraph where you see that 9-month backlog, you also see a mention that other companies were happy to fill the order: Fender, Marshall, Standel, Acoustic, or others. Customers who needed an amp weren't willing to wait - they're WORKING musicians - no amp, no income. So a lot of those backlog orders were canceled.
Ampeg was pretty slow to get on the train for current guitar sounds. Not until the early 70s did they finally make an amp with switchable distortion. The founder Mr. Hull HATED distortion. No problem with that, but he never accepted that rock was there to stay, and it was too late. And even when they did catch up, the sound they offered was unique, but just not "popular." Having cranked a VT-40 to eleven, it's a BEAUTIFUL sound!!!
I think the guys who like them tend to buy them and keep them. I've personally owned 6 vintage Ampegs, all made from 62-72. A Reverberocket, 2 Reverberocket IIs, a 66 Jet which never was loud enough & is still on my fixit list, a 68 Rocket II (but a Reverberocket II at heart) & a GU-12. My favorite was my GU-12 til my friend blew the OT, and I VERY regrettably gutted it without thinking. I still have the cabinet... My nicer Reverberocket II I sold to a jazzing buddy, who still gigs it with his archtop. It was a great sound, but as I progressed, it wasn't MY sound...
Another problem with Ampegs is the @#$%ing TUBES. Yes, they tend to last a while and most of my Ampegs did come with original tubes or NOS replacements, but I'd buy busted Ampegs cheap cuz it was less for the busted amp than a pair of 7591As. And where in the hell you gonna get a 6BK11 for my Jet? I managed to score some 6C10s, but that only fixed the tremolo, and even those are $100 each IF you can find them. 7027s, 7199s, 7591As, and even the cheap oddballs (6CG7, etc) nobody offers testing on. It's a bitch. Yeah, you can rewire the sockets, but the sound DOES change.
I really do love these amps, and they had a real commitment to quality, and didn't just copy Fender or Marshall circuits. They used cathdyje & paraphase inverters LONG after the other Big Names dropped them. They never issued an amp with a less-than-12" speaker until the SVT, and that wasn't exactly a cost-cutting measure, was it?