Based on your comments to the other answers, I understand your question better now, and your point is a good one. Consider the following:
You have a Gmail account with a password that is sufficiently complex, and you've had the same password for years. Perhaps you use an email client at home to check and store your email, and you use your phone client as well. You are traveling and lose your phone. You go to the mobile store and get a new one, and they transfer your number for you to the new phone. While trying to setup Gmail on your new phone, you can't seem to remember your Gmail password, so you reset it using SMS verification, and everything is fine, except that you know your email client at home is getting annoyed because it keeps failing to connect. It's not a big deal; you realize you'll have to enter in the new password when you get home. After stopping for a coffee, you have a moment of brilliance and you suddenly remember your old Gmail password. You try to change it back to the old one but now you can't! Dejected, you go back to your hotel, sit down on the couch, and find your old phone between the seat cushions. You think back to that moment when you were trying to remember your Gmail password... "Had I only been able to remember it, then I could have kept that awesome password. But I had a moment of forgetfulness and now that perfectly good password is wasted..." Of course, had you not found your old phone, you might have wanted to change the password anyway, along with all other passwords that were automatically stored on the phone.
So why doesn't Gmail let you reuse the password? Because they have no way of knowing you found your old phone.
In other words, they don't know why you changed it in the first place, so they simply assume you changed your password because you accidentally tweeted your old one to the entire world, just in case.