I don't really know either and I've been thinking about it a while. My first instinct would have been that it was a higher cost method of payment from the DH perspective, but I've heard that Google Checkout has the same fees as most merchants - though it may be that DH has negotiated lower fees with their credit card vendors, given their transaction volume.
Dunno, maybe it's some quirk of transaction processing with Google Checkout where it requires more merchant product information than other forms of payment? Perhaps we'll see Google Checkout for renewals later when DH has finished the extra coding, if that's what's required.
For those who want Google Checkout for renewals, it would help to add it as a Suggestion in the Panel. There are a couple of suggestions already for Google Checkout generally (already implemented) and for donations (more specific), but I think one specifically for renewals would be looked upon kindly.
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