There used to be signs all over, and of course, guides explain the rules.
This is a long tradition - it is a ruling by the Rabbis, going back centuries - till at least the time of the Crusades.
Jews are not allowed to approach the site which belonged to the Temple, apparently because they might offer prayers in the wrong place, or tread on the Holy of Holys. This ruling is expected to stay in place until the Messiah returns.
Israeli rabbinical councils have reaffirmed the “no prayer at Temple Mount” several times.