Why do some digital movies not have 5.1 surround sound available?

[MrHT]

There are a few digital movies that come to mind that are not available in 5.1 surround sound, such as “Teenage Mutant Ninja Turtles” and “Striptease.”

Both are Movies Anywhere so I can stream them on MA, ITunes, Vudu and Amazon. With TMNT, Movies Anywhere is the only app that has it in 5.1 surround sound but it’s out of sync. With Striptease, none of the apps offer 5.1. Yet both are 5.1 on Blu-ray.

What is with that? This is what makes me frustrated with going digital and making me cling onto physical media!