...because they failed to prepare for a 99 percentile event?
They weren't well capitalized. What gave you that idea? Just to preempt the "but DTCC said so" reply: https://news.ycombinator.com/item?id=28000185
>and continued to do so until the price was 1/10th of the high
Because the price isn't a relevant factor. GME being 1/10th the price just means that people would buy 10x more shares, leading to the same deposit requirements.
>...because they failed to prepare for a 99 percentile event?
Because they used the funds that their clients had deposited to buy stocks with as cash to make their margin business work. At least that's how I understand it.