Here's One Really Insightful Comment On The I-35W Bridge Collapse

I excerpted this from a comment on an earlier post of mine about the Minnesota bridge collapse.

"Next to 35W is the 10th Avenue bridge, completed in 1929. On the other side is a railroad bridge completed in 1887. Both are still standing while the collapsed 35W barely lasted 40 years. The Romans invented the keystone arch 2000 years ago and many of their bridges are still standing."

As the commentor asks: why can't an Interstate Highway bridge last more than 40 years?