A 50mm prime usually doesn’t benefit from optical stabilization as much as longer or slower lenses do. One common rule of thumb is that a 50mm lens can often be handheld at about 1/50–1/60 second on full frame, so camera shake is less of a problem than it is with telephoto lenses. Also, 50mm primes are typically simple, compact, and relatively inexpensive designs; adding IS would increase complexity, size, and cost. Prime lenses also tend to have wider maximum apertures than zooms, which already helps in low light by allowing faster shutter speeds. Across camera systems, stabilized primes have historically been less common for these reasons. So the likely answer is a combination of limited practical benefit at 50mm, plus the tradeoff of higher price and more complicated optics. It’s ultimately a product-design decision by Canon, but from a photography standpoint, IS has generally been seen as less necessary on a fast normal prime.