The Gemoro (bottom of 23a) queries which of two incidents was first, and concludes that the case of the Kohen who was stabbed was first, but the Chachomim thought it was a one-off (so did not change the procedure) until the incident of the broken leg, after which they institued a lottery.
Why could it not be that the incident of the broken leg was first, but the Chachomim thought it was a one-off until the incident of the stabbing?
Meir Eliezer Bergman
The Sfas Emes explains that the murder incident was clearly perpetrated by a Kohen Rasha, the type of person who is not commonly found trying to do Avodah (at the time of the incident). Chazal therefore did not deem it necessary to make a Takanah for the likes of such a person. However, for a Kohen to push someone off the Kevesh in a way where he could merely break his leg was something that was clearly possible to happen again, and warranted a change in practice.