The Gregorian calendar's predecessor, the Julian Calendar, was replaced because it was too inaccurate. It did not properly reflect the actual time it takes the Earth to orbit once around the Sun, known as a tropical year.

The Julian calendar's formula to calculate leap years produced a leap year every four years. This is too often, and eventually, the Julian calendar was several days out of sync with the fixed dates for astronomical events like equinoxes and solstices.

https://www.timeanddate.com/calendar/gregorian-calendar.html