The reason has to be that people would naturally just swap over to ult pings once they realized that alive pings were private. It is just the natural evolution and they wanted to get ahead of it.
^ Yeah this is the answer. A principle we're trying to adhere to is that nothing is persistently pingable so it doesn't just become a substitute for the alive ping. We are planning to restore some contextual uses of the scoreboard pings though - for example it's reasonable and constructive to say "Wait 10s for Malphite ult before we fight" but it doesn't help anyone to spam ping that Malphite whiffed their ult right after it was fired or died without using it, they really don't need you to tell them that they made a mistake.