Why Do Friendships End When Relationships Begin??

When my friends and I were single we hung out all the time, almost every day. We would go out to dinner, movies, bars, shopping, we were always doing something. Once we all got into serious relationships it seems like we have drift apart. I still hang out with some of my girlfriends we have dinner or grab a few drinks, but there are those few friends that i never hear from except an occational email or text. I really hate it! Although we now have our own busy lives; are we really that damn busy that we cant find time to meet up for breakfast, brunch, lunch or dinner? I miss the secret sharing, the gossiping, the laughing, sitting out on the patio drinking wine and being silly. We've all shared alot of great moments in our lives together, memories that I will keep with me forever. I don't want our friendships to just die and just fade away. I love my friends very much and its really hard to face the fact that some of them are drifting away from me. Just because we are all in a relationship does that mean our friends have to end??