Because it's free! You also get TLS(HTTPS) and theirs standard cloud protection. Their protection will hide the servers IP you potential attackers will try to attack Cloudflare instead of your server.
Cloudflare can also protect your site from other things such as spiders, scrapers and crawlers that want harvest data from your site.
With Cloudflare your site can handle much more traffic so you'll get less downtime. You also save a lot of bandwidth.
Because it's free! Tor tries to make the user anonymous and safe on the web. Tor gives the user an opportunity to browse the web without censorship.
Why not both?
Because many people use Tor, both for legit and not so legit usage, some crawlers will use Tor because it offer many IP-addresses. So if you try to visit a site which is behind Cloudflare you'll probably see this:
Make your site behind Cloudflare Tor-friendly!
First of all, use this script made by Donncha O' Cearbhaill; Automatic CloudFlare Tor Exit Whitelister
This script takes top 200 exit nodes and whitelists them. The number 200 is because Cloudflare only offers to whitelist this many. But this is fine because these 200 nodes represent around 90-95% of the exit probability.
It's really easy to use and you'll find your API-key in the settings menu at Cloudflare.com by login into your account.
There are also other things you can do while you're on Cloudflare. Navigate to 'Firewall' and set the 'Security Level' to Essentially Off and 'Challenge Passage' to 1 year, it should look something like this:
After this you should be set! I got some complaints about my blog a few months ago that some visitors got to solve a captcha and now there are no complaints!