I think the reason is the steroid hormone levels in blood stream. According to the theory, if blood hormone levels are steady the body reacts less to it. That reaction is manufacturing estrogen to ups and downs of this hormone level. With prop since it is active for 2 days, its easy to control and have a more steady level. With longer esters since there is hormone release from multiple injections at the same time ,it will be more unbalanced. on other words hard to control and calculate.