The reason is that many beginner users were copying and pasting the entire stylesheet into the editor, which ended up causing them a lot of problems if they didn't know very much about how CSS works.
The CSS is still accessible in a number of ways, including in a link on the Appearance → Custom Design → CSS page in the "Mode: Add-on" section. You can find it by clicking "Edit" next to "Mode: Add-on" in the Publish box.
However, it's often better (and easier) to find CSS using built in browser tools. Most modern browsers have a web inspector that you can access by right-clicking an element and selecting the "Inspect Element" option. In Chrome, it will show you the HTML for what you clicked on as well as all of the CSS that applies to it, including the order of the rules that apply and which ones are overridden by other rules. I help people with CSS all the time, and I find the browser tools indispensable.