Two major reason why I don't use glass beads for weighted blankets and weighted lap pads: Cost & Construction.
While glass beads have the benefit of being dense, they are very costly. Glass beads tend to be 2-3x more expensive than Polypropylene pellets.
Construction of a weighted blanket with glass beads can be a little tricky. Glass beads may pass through some fabrics, as well as transfer between pockets in the blanket. Lining the fabric with a muslin/cotton and reinforcing the stitching is recommended.
Bottom line, there is no real benefits to using glass beads for a weighted blanket unless you are limited on space.