Why Were These Two Performances Dubbed on Tonys?

In particular I am curious about Angela Lansbury's dubbed performance the year of Sweeney Todd and Robert Preston's dubbed performance that I believe aired on the Tony 25th anniversary broadcast.

IS there a way to ask Ms. Lansbury why she did not sing live that night? Who made the decision and why?
(I realize the reason Len Cariou is not in the scene they did from Sweeney is likely because it might have been a bit insulting to ask him to participate in "The Worst Pies In London" being broadcast on the Tonys--a song that he had no lines in.)
The same with Mr. Preston's dubbed performance. Anyway to find folks involved at the time who decided for him not to "sing live?"

I would be delighted to hear a definitive answer from someone who really knows.

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the charachters of Sweeney Todd and Mrs. Lovett are the only ones onstage during The Worst Pies in London. The original Broadway cast had Len Cariou as Sweeney and Angela Lansbury as Mrs. Lovett.

During the Tonys they presented "The Worst Pies in London" as the number from Sweeney Todd. However, Mr. Cariou did not appear onstage as Sweeney. Another actor sat in the chair where Mr. Cariou as Sweeney would have been sitting.

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oh ok, sorry - i wasn't paying attention to what your wrote...

Most of the Tony performances of nominated shows in the early 80s were dubbed. I don't know why, but you can see it in other performances. The most obvious I've seen were that one from Sweeney Todd (when Angela misses the first cue) and Jim Dale in Barnum.

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