Why Airfares Are High on Airline's Official Websites Compare to Other Travel Websites?

When I tried to find out cheaper rates I thought buying from the official websites would be cheaper but I was wrong. Some travel websites like priceline.com offers same company's flight for whole tour then also the rates are cheaper compare to that airline's official site.Why is it so? And there is a huge price difference for international flights.Can anybody please tell me why is it so?

Suggestion:

I don't find that to be true in most all cases. It may be true on certain flights and certain airlines, but I just checked the same international flight on Priceline and compared it to the same flight on the airline website, and found the flight for almost $100 cheaper on the airline website.
Also, many airline websites offer low price guarantees that will match or even pay you a small amount if you find the airfare cheaper on a travel website. It also may be a ticket with a different fare class that is highly restricted that you see on Priceline which may not be eligible for frequent flyer miles, may have high fees to make changes, may not allow any changes, may make you purchase another ticket entirely if you miss a flight or try to make schedule changes and good luck should you try to amend a ticket using websites like Priceline.
I think it depends on the destination and the airline, it may be true in some cases, but definitely not all.