Using the Excel.Application com object in Powershell won't save a file through the task scheduler for automation.
It's likely a DCOM permissions issue. Automating Excel is sometimes fraught with peril...
The only way I've found around issues such as this is to set Excel to run as a specific user through DCOM permissions.
Open Component Services (Start > Run, type in dcomcnfg)
If you're using 32bit Excel do this instead from a command prompt:
C:\WINDOWS\SysWOW64>mmc comexp.msc /32
Drill down to Component Services > Computers > My Computer > DCOM Config
Right-click on Microsoft Excel Application and choose Properties
In the Identity tab select This User and enter the user that is running the task in task scheduler