It's hard to explain why. After all, Bold 9900 is a 5 years old antique of antiques, and of all antique phones. it's a Blackberry LOL.
There is no better person to explain this than Steve, and in a way, his Avatar tells it all!
People who use or stick to the Bold 9900 are not dumb but I would say they are rather more experienced people who can sift through nonsense and see the real value of communication device. No, it still does not answer your question. To me, it's a more refined usability than other fancy devices. In my opinion, if you really really analyze the value of communication device, and after compromising all your past belief, I found the Bold 9900 has so many strengths that other fancy ones do not have. Specifically, adding what Steve said, it's mostly about its user interface whic makes you more efficient in most business scenes. Yes, its all about communications such as email, text and yes, phone! But beyond that, it is what's hidden under antiquated menu system and way to manipulate it with keyboard and tool belt. Most everything you normally encounter in communcation situation is right there at your fingertip. Yes, it's browser is slow and all that, but that's what I say is "compromise" You can't have it all (I still wonder why there is not a single smartphone that has it all), but you weigh in pros and cons after "using it extensively", you will learn where to compromise in order to benefit from strong points of the Bold 9900. I know I am still not responding to your very legitimate question well. It is difficult to explain without going through each specific occasions where Bolds are superior.
Re Classic. this was a brainchild of John Chen when he became CEO and immediatelly got a year full of complaint from corporate executive and others for throwing away BBOS and came up with BB10. There, there is another reason why large number of people are still sticking to BBOS, not just for the security of BIS/BES. So, Che came up with Classic, but alas, it was no "Bold 2", because it still did not duplicate the BBOS UI, replaceable battery and so forth.
When BB10 came out, there was a huge uproar about it. I have a feeling that BBRY, even including Chen himself, still does not understand why there is such a huge population of Bold holdouts. I do not understand why they do not understand it.
One Canadian newspaper (Globe & Mail) famously coined a word "moments of little delight" that was taken away from BBOS when BB10 came out.
It's an interesting read. I cannot immediate find the article by Googling keywords, but this may be the one.
How BlackBerry blew it: The inside story - The Globe and Mail