What? How was it not?
A normal year is defined as 365 days. However, if you measure the exact amount of time it takes for the Earth to orbit the sun, the number is actually 365.242199 days. By adding one extra day to every fourth year, we get an average of 365.25 days per year, which is fairly close to the actual number. To get even closer to the actual number, every 100 years is not a leap year, but every 400 years is a leap year. That brings the average length of the year to 365.2425 days, which is very close to the actual number.
Putting all of these rules together, you can see that a year is a leap year not only if it is divisible by 4 - it also has to be divisible by 400 if it is a centurial year. So 1700, 1800 and 1900 were not leap years, but 2000 was.