A lot of horses don't like beet pulp and won't eat it unless it's mixed with something else. Keep in mind that rats won't even eat it, so it can't be very appetizing in itself.
Adding a handful of oats or other well-liked treats can help. If you had chunks of apple or carrot, most likely the horse will root through and pick out all the chunks. Try something finer. Or pour a little molasses on it and mix in.
Be sure it soaks for a LONG time - at least a couple hours, and use hot water and let it sit and steam, covered is best. Soaking it longer and really steaming it helps to make it more palatable too.
Another tip to make it more palatable - add only enough water to puff it up and soften it, not enough that it's sloppy or wet, with loose water left at the bottom of the bucket. Putting it in a flat rubber feed pan that sets on the floor, can also help.