The Builder wants the project to close on their schedule (barring they have the certificate of occupancy from the city and that the home is substantially complete). If the closing date is delayed past the month end then the Builder will have one more payment on the construction loan. The construction interim loan is a much higher rate than a conventional loan.
If the Construction Company is listed in the stock market then they have investors to keep happy. They want to close so many units per month nationwide and by all means close as many homes as is possible by January 31st each year.