I'd say it's a natural result of structure. Social teams can be small and internal because they can choose where to spend their time - they probably ignore 99% of the tweets they see ("Customer support can help you with that. Thanks!"). Hence they can focus on stuff that actually needs escalation, and being internal they're more likely to have met somebody who'd want to know about an unusual issue, etc.

Whereas phone support is the opposite - it's designed to be handle the 99% bulk of requests that you wouldn't want to be escalated, so it's not too surprising that it's not as good at handling outliers.