Because it stores all your important information in one big file. Yep, that's right. ONE BIG FILE. It's called a .pst file, and it contains everything you've ever entered into it.
So that if something happens to that file, you have now lost all of your contacts, all of your notes, all of your calendar info, and all of your email. ALL AT ONCE.
And don't think it won't happen. Hard drives crash. Sodas get spilled onto laptops. Oops, sorry, your stuff is gone.
I have been trying to wean my husband off Outlook for years. I first started nagging him about the issue a few years ago when his .pst file was a litle over 2GB. It was behaving erratically, refusing to open, refusing to close, and being very, very slow. That's when I discovered that that version of Outlook wasn't designed to handle files over 2GB. He was supposed to be archiving his older emails to keep the file size down. But he didn't want to do that because he likes having them close at hand.
So we upgraded to a newer version of Outlook. Problem solved, in his mind. But I started to worry. And nag, "Please switch to a different program". But he was resistant because he was used to Outlook, and he didn't want to have to learn a new system.
And so the file keeps growing. And growing. And growing. It's now a 5GB ball of wax. And to me it looks just like a time-bomb. It's too big to back up to a CD, and he doesn't have a DVD burner. I back it up to my computer over our wireless network and it takes for-ever. And since it's a pain in the butt to do, I don't do it as often as I should. When it goes he's probably going to lose a few weeks worth of stuff. And I'm going to have to bite my tongue.
So, if you're one of those people who has an automated backup program (anybody?) or if you back up your Outlook file to an external device every day (anybody?), then by all means,use Outlook to your heart's content. It's a terrific program, very sophisticated, very versatile, and you already know how to use it.
But don't come crying to me when you lose all your stuff.