The only reason, other than outright compatibility issues, is price.

The licensing costs for SQL Server 2012 are different than 2008R2 and less. For most situations I've seen, they're more. So, going with the older version could cost less, at least in the short term. But then you're installing six year old technology as a starting point. It's more likely to age out of support, not have necessary updates, all sorts of other, hidden, costs that may preclude that short-term licensing cost gain.