You actually have the answer in your question - because they're so popular.
AP is a family company, not a big mega-corp with millions to dump into increasing production. There are also potential pitfalls with a small company scaling up - Google for general discussions. I had a multiple cage order with them that I called to see if I could pick up at the 2015 spring NARBC in Tinley Park, but they weren't going to be there because they had too much production work to do.
BTW, for those who haven't checked their website, a T8 is a quick ship item, about 3 weeks.
Currently have a T10 on order, debating on finances as to when to add another.
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