Because I wanted it to stay separate from the rest of the WordPress theme on my site. That also makes it more portable if I need to setup the landing page on another domain (if traffic cripples my main site).

Since it is just a single page site, I didn't feel the need to do separate files.

I should add that I didn't code it. Got too busy with the book launch so I hired it out.

Thanks.

For what its worth there is 1 theory for SEO that the code to text ratio on webpages (not apps obviously) may play an extremely minor part in ranking. If you served the css from a cdn the page would be faster (rank higher) & better optimized (rank higher).

I can't say it will make a difference, but it might.

Also speeds up the page. Just one HTTP request, instead of two.