Having put a lot of effort into backstory or worldbuilding or character development, there is a natural desire to get some return on that investment beyond just knowing how your setting works today and how your characters interact now. Working in the backstory therefore becomes almost compulsive with some authors, inevitably leading to bloating the manuscript way beyond what the current story can carry.
If the backstory is actually that good, write it as a separate story and sell it to a magazine, so that when the novel comes out, there is already a market for that character and/or setting. Readers love when they know backstory from having encountered these characters before, and will often seek out other stories with the same world or characters. Or write it as series with prequels and sequels galore. But today's book has to stay focused on the current story, unless there is a compelling reason to risk going off message. Trying to fit all of that into one book inevitably distracts from the current action and leads to a bloated manuscript.