(As someone who has been in smart-metering for the past 6-ish years, and been s/w architect for both elec and gas smart meters, the situation is that the residential meters (i.e. high unit count) demands the cheapest and smallest processor that the s/w will run on. Industrial meters OTOH, are relatively high-cost and can justify a Linux-based device (and indeed there are examples out there that have gone this way). Tho due to commonality with the residential meters, it is more efficient for them to be based on the same hardware generally.