Why is BookNow not mobile friendly (responsive)?
It actually is, but it cannot be responsive within an iframe that is embedded in your website. If you link directly to BookNow then it will work well on mobile devices.
One trick that you can use is to show the BookNow app embedded in your website when the customer is on a non-mobile device and to hide a direct link to the BookNow app. With a CSS rule you can then set to hide this embedded app and to only show the link to the BookNow app when the visitor is on a mobile device. You can get the direct link from the iframe src.