answer:
because there is a difference between (1) a genuine failure where the company makes an honest effort to bring the product to fruition, and (2) one where the product 'fails' because the company spends all the money on something else, while blatantly lying to their backers about the progress being made.
the vast majority of crowdfunders in scenario (1) would not be asking for a refund. The Vega+ however is a case of scenario (2), and backers obviously feel entitled to their cash.