Other than you having a problem, nobody can give you a definitive reason as to why. Given the circumstances and the steps already attempted, you would be best advised to seek assistance directly from Apple.
The potentially best avenue would be to visit the Genius Bar at your local Apple Store - where the technicians can try some hands-on diagnostics. If this isn’t possible, you can directly contact Apple Support using the Contact Support link at top-right of this page; if contacting Apple Support, it follows that direct access to - and diagnostics of - your iPad won’t be possible.