Because from what he told me, the builder that put that engine together had racing in mind, and I'm willing to bet that he was a little more generous with the bearing clearances than a person would have with a stock engine. Not necessarily loose, but on the loose side. For example, if he was looking for a clearance between .0005" and .0025", he likely went with .0025". I don't know this for a fact, but I do know that the builder was experienced, and my experience leads me to believe he might have done things this way. If he did, than this engine will be happier with 10w40 during the warmer months, IMHO.
Take care,
~Chris