Decentralized services, like email, have a big advantage for users. I can send email to anyone that I have the email address of. The advantage for me is that even if my friends use a different email provider, I can still send them emails. It would be a huge problem if I'd need an email account with every provider that I wanted to send emails to. That I can use one account to send email to anyone that reads email is an advantage for me.
Lets assume for a moment that you could only send email to the people at the provider where you're a customer. My friends would need to be customers at the same provider as well if I wanted to send an email to them. If I wanted to send email to someone completely different, maybe at the other side of the globe, it would probably be impossible. It's important that everyone you want to send email to is a customer at the same provider as you.
Now lets assume that another provider starts to offer a new and cool email related service, maybe they send a free song every week. Now a few of my friends will move to this new service, they like the free songs. And I can't send email to them anymore. So, now I have to move as well. And then a few more people move. And now the parents move and the grandparents move. They don't like the songs, but they want to send email to their children and grandchildren.
Every time a new and better provider appears people will move, first slowly, but after some time, faster. The centralized structure of this email system makes people move in groups and because some people are part of multiple groups, they'll make other groups move as well, until everyone has moved.
Another disadvantage that appears is that a centralized system needs to be one size fits all. And while this somewhat works for t-shirts, this doesn't work at all for software and websites.
Luckily email doesn't work this way, but some websites do. For example, it's impossible to have multiple Twitter providers. Their terms of service don't allow it. A new service that provides micro blogging can't work together with Twitter. So, if at some point a new service comes along and Twitter isn't the hot new thing anymore, then people will flock to the new service, because everyone is.
On the other hand, if Twitter would interoperate with other services, then there is no reason for people to move, because they can still send messages to their friends and everyone they care about. The people who like Twitter will stay and the people who don't like it, will move. You'll only switch if a different service better fits your needs.