Why can't February have 29 days in a non leap year and 30 in a leap year?

That would create too many days in a year. It takes about 365.25 days for the Earth to orbit the sun. So once every 4 years, those fractional elements are combined to make one day and that is added to February, as it is the shortest month. If it was always 29 days, then there would be 366 days in the year, which would put the years out of sync with our orbit of the Sun. That would affect our years and our seasons over the course of time as those extra days mounted up, so it cannot be done. It has to be kept at 365 days and then every 4 years we combine the quarters to add in the extra day.