Presumably because it's cheaper. Ours was also originally in home; the utility recently came through and replaced all of ours with exterior units, likely at great cost (horizontal drilling machines aren't cheap)

I suspect the tragedy in Pennsylvania [1] was the catalyst. There are lots of potential issues with keeping these things indoors.

1. https://en.wikipedia.org/wiki/Merrimack_Valley_gas_explosion...

Meter still looks very accessible to me

Sure, but somebody has to be home if the gas company wants to look at it.