1. Her ARROGANCE. For all her excellent qualifications as a policy wonk, she was an awful candidate. She was an awful candidate TWICE and she lost TWICE BECAUSE of her arrogance. No one is going to tell Hillary Clinton how to run her office, or so it would seem. So, when she was told to move everything over to a federally certified dot.gov server, she basically muttered something like, "Yeah, I'll get around to it....whenever"...which is something like "maybe when it gets to be so difficult to do otherwise that I HAVE to", because of the famous Hillary Clinton arrogance.
2. And this might be the most IMPORTANT reason why she stubbornly clung to her private email server:
---Just like Trump, Hillary Clinton DOES NOT KNOW HOW to use a computer.
Oh, she can hunt and peck on a word program maybe, but when it comes to advanced office use of a computer, Hillary is almost exactly like Trump. She "has people for that". Hillary knows how to use her "Blackberry" and the one she was fond of stopped being approved for federal use many years ago, but she has stubbornly clung to it, and the email server that it requires in order to function.
The main reason why, the REAL reason why she refused to move off her private server is because she could not get it qualified and approved to be moved to a dot.gov domain with federally approved security. Her server and the antique smartphone she loves so much aren't secure enough.
But she didn't care because she did not want to LEARN HOW to use a new piece of technology. Hillary, Trump, all these old dinosaurs, are all the last of a breed - OLD FARTS who cannot grasp how to master new technology.
My oldest brother is exactly like her and Trump in fact - HE TOO, stubbornly clung to HIS Blackberry until it just plain stopped working properly. And now he complains 24/7 about his new phone, and he CANNOT use even the simplest computer...not a Windows machine OR EVEN a Mac. He can't even master how to use a Mac, which is in reality a bit simpler to use than a Windows machine.
That is the real reason why the whole email thing happened, laziness, arrogance, dislike of new technology.