Because mass uploading of photos via email using Picasa (which is what I did with iPhoto for 2000 or so photos) s*cks because you can't enter tags. And by not tagging your photos, you are not as much a part of the Flickr community as you could be (and as most people want to be!).
From Gmail, Picasa and Flickr.:
With the recent release of Picasa 2.0, you can now email directly from this great image management software. When you email from Picasa, you can tell the application to automatically resize your photos before sending them. So uploading to Flickr has become much easier.