Well, SSDs have hardware to track allocations anyway. The trim command just manipulates that.
Regular hard disks are basically big platters of bits. They don't have any allocation tracking. Because implementing a generic (filesystem-agnostic) trim would require adding another software layer and allocating space to bitmaps, I think it's unlikely the benefits would be worth the complexity.
But who knows! It's a little hard to predict the future of storage right now.