Wire fraud is no less serious punishment wise. The main factor is probably the operation cost, as sending scam mails is more expensive per recipient and scams usually have a fairly low yield.
> Wire fraud is no less serious punishment wise
That might be true in principle, but definitely not true in practice. Cost might be the factor and the related facet is volume -- there's just no reasonable way to enforce it to the same degree as postal fraud and as you said, the price of the stamp itself is a deterrent. I get scam texts all the time; I've never gotten a scam letter.I appreciate that all of my official tax and government documents continue to be delivered via physical mail.