Congratulations on your acceptance! That is an excellent school and you shold be proud of your hard work to get there!
There really isn’t anything else to do. The reason for the rejection is simple: college applications are always a crapshoot. The same kid can get accepted to Harvard and rejected from Yale. Into Duke and booted from Vanderbilt. Accepted to Amherst and rejected from Williams. There’s not some objective point at which you’re “good enough for schools above X ranking.” This is a subjective process. Depending on the mood of an admissions officer, or the precise things that a particular school is looking for that year, you might be a perfect fit, or an immediate rejection.
Be excited of your success and good luck at college!