I think the biggest issue here is that your question seems to assume the Hashemite Kingdom of Jordan is a democracy, and that treaties are typical entered into with democratic input.
While Jordan participated in the Arab-Israeli conflicts previously, Egypt signed a peace deal with Israel in 1979, and Jordan followed suit in 1994. The question of why the political will to continue fighting waned is an interesting one that I don't have the precise answer to. However, the answer to the question of why Jordan signed a deal contrary to the wishes of a majority of its people is simply than it's not a democratic state, and the monarchy is free to do as it pleases.
For decades the government of Jordan had maintained independent communications with Israel, avoiding direct confrontation with Israel whenever possible, and cooperating with Israel against common enemies when useful.
During the 1948 war, for the most part Jordan's posture was defensive, despite possessing the best equipped and organized field army in the Arab world and sitting in a strategic position on the West Bank where it could easily threaten the "narrow waistline" of Israel, while Israeli forces were occupied by Syrian and Egyptian invasions. At the close of the '48 war, Jordan was unquestionably the best off of the Arab states, having gained the west bank and Jerusalem's holy sites at little military cost.
In the '67 war, Jordan made the mistake of allowing themselves to be drawn into the fighting, and suffered catastrophic defeat, losing the most prosperous parts of the Kingdom and absorbing hundreds of thousands of Palestinian refugees. This greatly destabilized the Jordanian regime, and the next few years saw a vicious "civil" war between the PLO and the Hashemi state, during the course of which Syria intervened on the Palestinian side, but were intimidated out of the conflict by the threat of Israeli and US intervention on the Jordanian side.
King Hussein of Jordan recognized the value of maintaining the regional balance of power. Upon learning of Syrian and Egyptian plans to attack Israel in 1973, he took the bold move of flying to Tel Aviv to meet Israeli prime minister Golda Meir and warn her of the incoming attack. (His grandfather, King Abdallah I of Jordan, had also met with Golda Meir to plan for regional war - over 25 years earlier, before the '48 war)
So in answer to your question, the Jordanian regime, because it was ruling over a fractious mix of local peoples and Palestinian refugees, saw its best chance at survival in cooperation with Israel and the west and avoidance of direct conflict.