Interstate Highway Numbers - AASHO

In the Straight Dope Classic on interstate highway route numbers Cecil mentions:

. . . which was devised by the American Association of State Highway Officials (and I’ll bet that’s an acronym you don’t hear pronounced too often).
The group’s name is now AASHTO (pronounced Ash Toe) with “and Transportation” added in as part of the title. A quick Google search brings up AASHO listed on technical papers in the 1960s. I don’t know when they made the name switch but it amuses me to think of someone starting off a presentation: “Hello, I’m with Ass Ho’e” and thinking “Damn, we really need to change that name.”

Why wouldn’t you pronounce the “sh” as a single sound?