Artwork for silkscreening needs to be able to be separated into colour plates. Each colour in the artwork will be printed on a separate screen, and is applied to the t shirt one colour at a time.
While it is possible to separate a bitmap image (like a Photoshop file) into colour plates, it is very difficult. Photoshop files rarely have only one shade of each colour. There will often be variations of shading, and the blocks of colur will always be fuzzy and pixelated around the edges. When printing on paper, these colour variations are printed with a technique called halftoning--a grid of very small dots to make the colour in that area appear lighter without actually using a different ink colour. However, since silkscreen printing is done using screens that are already grids of very small holes, attempting to print a halftone pattern through one is very difficult and usually disappointing when compared to artwork that's displayed on a computer screen or printed onto paper.
It's much easier to print artwork that's created as vectors, using software such as Adobe Illustrator or CorelDraw. Instead of being made up of pixels, vector art is made up of mathematical objects; lines and curves and shapes. These objects can easily be created to be one consistent colour with no blurring or blending at their edges. They can also be made to overlap each other in different ways, which is exactly what happens when the screens are being printed onto t-shirts. And, as a further benefit, vector art can be made bigger indefinitely, with no loss of resolution.