This is the entire dataset I could find of merchants that accept both, find me more and I'll gladly report on them. Theres no selection bias, in fact most of these merchants are biased towards other coins.

Monero is used more because it's safer and easier than Bitcoin. People don't want to pay capital gains tax on every transaction or have their whole wallet doxxed to each merchant as they would have with Bitcoin. That last bit is my analysis but the numbers speak for themselves.

Once again, please find me more data that's all I can find as of today.