Cold Feet
Sometimes there’s no other reason than last-minute jitters for calling off a wedding. The OP’s mother was single for a long time before meeting an amazing guy that the entire family loved. After dating for 6 years, he popped the question and she said yes.
They planned the whole thing and she even got a dress but one day before the wedding, she got cold feet and told him she couldn’t do it. As it turns out, even though he was a great guy, he wasn’t very motivated and had no goals in life. That ended up being a deal-breaker for the OP’s mom and she decided they would be better off as friends. Fair enough.