As somebody who didn't do this, let me offer the reasons (good or bad).I'm only a DM and not an instructor, although one day I may pursue an IDC. But my opinion is that IDCs should be done in conditions/locations where the person is going to be working or hopes to be working. I don't know where Snow lived and dived before her IDC, but it seems to me that learning how to teach students in warm water is very different from teaching students in cold water when both the instructor and the students are wearing two-piece 7mm suits (or drysuits), hoods, and gloves and visibility and mobility is limited. But this is my opinion, so I'm curious to hear what the instructors out there have to say.
Where I live, nobody is making a living as an instructor. For US tax purposes, I lost around $500 teaching SCUBA last year. All to say we have day jobs and teach more as a hobby than income source.
At the time I did the IDC, there really wasn't more than one option a year to do it in my neck of the woods. And my day job has inflexible scheduling for about 40 weeks per year. (I can't take vacation time, and I better not get sick...)
That meant I'd have to travel. The only cold (-ish) water sites that had regular IDCs were in California, and even there they don't have the lousy viz we sometimes get saddled with. Moreover, if I did one there I'd pay more for housing myself than the cost of the IDC.
So to the tropics I went, because housing there came with the course, the CD was highly recommended (and amazing) and the course included emergency oxygen provider and nitrox instructor training, which I really needed.
That said, nearly all the instructors in my neck of the woods DM'd for a substantial time before the IDC. No zeroes to heroes to speak of. We'd see how things really are in cold, murky water. We'd thought about how to have students arranged during a tour. (Tip: Single file may work in the tropics, but it's V-formations around here.)