On: Why can't I get people to participate in this market research?

Really its simple math:

The people you want to interview charge around $100/hour (or more). You're asking to buy >$50 worth of time for a $25 gift card.

Change the offer to $50 payable via Paypal, commit to it taking 30 minutes or less, and you'll probably get some responses.

Out of curiosity -- would you be more willing if it was a chance for a single participant to get $500?

Not really, given the odds it would be worth less than the Amazon card.