Why are large companies challenging convention and turning to external service providers?

Many Cloud Computing pundits have predicted that the early adopters will be largely comprised of small and mid-sized businesses.

Some new data from Forrester suggests that won't be the case.  

According to Forrester's Frank E. Gillett in Conventional Wisdom Is Wrong About Cloud IaaS, one out of four large companies plan to use an external provider soon, or have already employed one. Furthermore, we learn that 33 per cent of large companies plan to use a service provider for Infrastructure-as-a-Service, while just 24 per cent want to run their own "private" clouds.

The question is why. 

Why are large companies challenging convention and turning to external service providers?

Mezeo chaiman Gray Hall answers this question on the Cloud Storage Strategy blog.