The runas command does not allow a password on its command line. This is by design (and also the reason you cannot pipe a password to it as input). Raymond Chen says it nicely:
The RunAs program demands that you type the password manually. Why doesn’t it accept a password on the command line?
This was a conscious decision. If it were possible to pass the password on the command line, people would start embedding passwords into batch files and logon scripts, which is laughably insecure.
In other words, the feature is missing to remove the temptation to use the feature insecurely.