Because it causes movie theaters to gain less profit then they normally would, if the movie comes out on dvd nearly 2 weeks after it is released it means movie theaters lose business due to most people would just wait and buy it on dvd
Example:
Movie released, we'll call it TheMan. So TheMan is open in theaters, not all people see it on opening day... and usually movies stay in the theaters for several weeks, sometimes months. So if TheMan is put on DVD only 1 month after, many theaters have no point in running it because half the people would just rent it or buy the DVD to save time and money.