Because with CloudFront there are dozens of origin servers around the world, and problems like the ones I experienced could be caused by a DNS server somewhere putting someone in touch with an unavailable server. S3 serves from one location (the location where you created the bucket) and hence is less likely to fail in the same way.
Yes, but if that one S3 location is having troubles, all of your users are affected, not just some of them as when CloudFront has trouble at a single location.
Why are you confident you will serving directly off S3?