Why are there fewer size options for Full Colour Envelopes?

Because these envelopes are printed first then cut, a die is required. A die is a cookie-cutter like tool with blades embedded into a wooden block, and therefore the size that can be cut is fixed and a new die is required for each new size. The options available are the most commonly ordered sizes and the dies are already made. As new dies are made, new sizes will be added.