Why Can't Unity Support System.Speech?

The title says it all, why doesn't unity add support to System.Speech

 

Sorry for necro, but I would like to know this too.

 

The mono version that unity uses does not include System.Speech.
https://docs.unity3d.com/400/Documentation/ScriptReference/MonoCompatibility.html
System.Speech is only available from .net 4.x and up but the mono version unity uses is a mix of dotnet 2 and 3 but not 4.
https://msdn.microsoft.com/de-de/library/gg145021(v=vs.110).aspx
Maybe for windows phone it may be available as that platform uses .net directly and not mono.