Besides for all the reasons already mentioned there maybe a medical reason as to why a horse only has front shoes. For example my 29 year old appendix gelding has navicular so he needs aluminum wedge shoes to help counter act the effects of that. Several years ago my farrier started putting wedged egg bar shoes on him. I never questioned why but figured he had his own reasons for doing so. Then back in 2013 my horse screwed up a front tendon and vet said that the egg bars were perfect for him due to the extra support that they give him. Ever since I retired him 10 years ago I have had him barefoot in the back except for a year when one of the hoof walls in the back collapsed on him and vet said he needed special shoeing in the back so had that done had gave enough time for hoof to grow out then had back shoes pulled again and no more issues in the back.