Creative Commons licenses are legally enforceable and free to apply. Through the US Copyright Office, pattern designers could end up spending lots of money registering each work especially if they come out with a new pattern weekly or monthly.
A prevalent concern among pattern designers is larger companies reusing their patterns with attribution or compensation. For example, the Creative Commons Non-Commercial Share-Alike License means that you, as copyright holder, can sell your pattern in addition to having it on a blog freely but if someone else uses your pattern, they have to share it freely, attribute you, revise it, and cannot sell it.
Sometimes, a pattern does not fall under any copyright restrictions because it consists of basic stitches and/or construction thus lacking the uniqueness and ingenuity needed for copyright. CC licensing can only be applied to copyrightable works. Good alternatives to think about when a pattern itself cannot be copyrightable are licensing the promotional photos, figures that were created for the project, charts, instructional videos,etc.
Office, U. S. C. (n.d.). Fees. Fees | U.S. Copyright Office. https://www.copyright.gov/about/fees.html