This is just my supposition, but I'd give 3:1 odds that it's at least basically correct: most of the people really wanting / needing the Pro-1's advanced capabilities and willing to pay its fairly high price would much prefer a printer than can handle 17 inch wide paper, for only a little more money and a slightly larger size. The Pro-1000 was and is such a printer. And from the lower end / those who really only want a 13-inch printer, the Pro-10, introduced not quite a year after the Pro-1, has to have cut into the lower end of the Pro-1's market.
The Pro-1 was what you might call a 'tweener' (from 'in between'), meaning that it didn't fall into any of the usual pigeonholes. It was a clear step up from Canon's own Pro 9500 Mk. II and Pro-10 and Epson's R3000 and P600, but it couldn't print larger like the aging Canon iPF5100 and Epson's 3880 and P800. Sometimes tweeners are very innovative and popular; other times they reinforce why the pigeonholes exist.
IIRC, the Pro-1 offered more on the black and gray spectrum than any other OEM inkset, with three grays (dark, medium, and light) in addition to photo black and matte black (and IIRC, the Pro-1000 dropped the darker gray to add blue). And FWIW, it was 4800x2400 dpi (like the Pro-10 and Pro-100), while the Pro-1000 is 'only' 2400x1200; can that make a visible difference in small prints / close viewing? I don't know, but it would be interesting to see a truly scientific test, which I've never seen. And the Pro-1 used 36 ml ink tanks, a big step up from the Pro-10's 14 ml cartridges, but well short of the Pro-1000's 80 ml cartridges.