Mmm... just some wild guesses
- You had arty and they didn't. Look at the losses you produced to them at distance. They could not respond until they thing get to short range.
- You had cavalry to shock (in addition to the cohesion lost by the guns) and specially pursue them when they tried to retreat, causing them more losses
- Half their combat troops were crappy milita. You have 100% regulars
- You had better leadership
- The battle happened on clear ground with good weather. Perfect setting to your arty and cavalry to do the job.
If they had defended on forest or wilderness and/or bad weather, things would had been more even i think.
If you want the full battle story, check the battle log in here
C:\Program files\Wars in America\WIA\Logs
Its pretty long and detailed, beware! But interesting !