Why doesn't my iPad support Low Battery Mode?
'Why' is always difficult to answer.

That it doesn't appear to work on iPads would seem to be of concern to many people, but there doesn't appear to be any answer as to why.

See Where’s The Low Power Mode in iPad, iOS 9 Users Want To Know or
Low Power Mode Not Available on iPad in iOS 9: No Solution Till Now &amp; others on Google. No-one seems to have any answers.