Actually a conservative professor in tweed *did* do that. His name is Arthur Herman and he wrote a biography of Joe McCarthy that makes a similiar attempt to rehabilitate the late senator. And you're right...it probably hasn't sold as many copies in it's publication history as Treason did in it's first week.
That said, I like Herman's book better. He's a little more willing to show McCarthy's flaws than Coulter is and is thus a little more credible. For instance, he talks about Whittaker Chamber's negative view of McCarthy and Ann Coulter omits that. It is a more nuanced treatment.
But then complaining about a lack of nuance in Ann Coulter's books is a little like complaining about a lack of pork chops in a kosher deli.
In her acknowledgment she thanks ..."M. Staton Evens, the world's leading authority on Senator Joseph McCarthy, who gave me---gratis---original research for my BRIEF version of a topic he will cover in meticulous detail in his forthcoming book..."
Ann doesn't claim to be the "utmost authority". She's a polemicist, who researches her wisecracks! She's a cartoon. Like Madonna, Dolly Parton, she is aware of her attributes and markets it. She's getting the message out to the less studious American.