It's print, but this is why we prefer RGB over CMYK?

That's easy; RGB! This color space is much bigger than CMYK, and the bigger the color space of the image is, the more accurate the printed colors will be!

Color Crash-course:

A color space is the amount of possible color nuances that can be created using a given technique or algorithm. The computer screen creates the colors we see using tiny Red, Green, and Blue lights (RGB) and can, using countless combinations of these three colors in varying strengths, create tens of thousands of different colors.

When we print an image it is created using four inks; Cyan, Magenta, Yello, and Black (CMYK), tiny droplets of which are printed too close together to be seen as individual dots, and it is this "optical mixing" which creates the appearance of a wide range of colors. The size of these droplets defines the dots per inch (dpi) and overall quality and vibrance of the image.

When printing on textile, due to the challange of the material, 6-8 ink colors are used; the four CMYK colors plus a few more to print dificult colors. Because textile absorbs more ink more than paper does, the resolution and overall image quality is somewhat limited. The darker a color is the more ink it takes to make it, and this ink gets absorbed into the fibers, which causes the color to bleed. To keep images from bleeding, the color space gets slightly reduced, to ensure that all the colors can be printed cleanly and accurately.

Let's say you are creating a nice design on your computer and you find a color, for example a certain shade of green, that you really love. It is possible that this color, shown on your RGB screen, cannot be exactly replicated using CMYK inks, so you save your image in CMYK, causing that lovely green to change a little to fit into that CMYK color space. It's very possible that the CMYK colorspace you chose is also not exactly printable, so again, the next most suitable green will be used when the image is printed, and over multiple steps your lovely green will change.

When you create and save your image as an RGB file, this color may change, but only once, and the smallest amount possible. To keep your colors pretty, use RGB.