Wrong guess. It was because the Jordanians, Egyptians and other Arab countries were still promising to conquer Israel and "drive the Jews into the sea."

And the Jordanians treated the Palestinians so well that they went to war with each other not long after.

In this case you are the one who is wrong. At least about the intentions of the Jordanians...

"The real question about the Jewish Agency-Hashemite agreement is not whether it existed but what happened to it in the course of the Israeli-Jordanian battles of May-July 1948. Clearly, it partially unravelled in April 1948 under the impact of the Haganah's military successes, the gradual disintegration of Palestinian society, and the Dayr Yasin massacre. 'Abdallah felt unable to stand aside or to break ranks publicly with the other Arab leaders.

But did he really renege on the agreement? ...On 10 May 1948 Ben-Gurion sent Golda Meir to meet 'Abdallah once again in a last-ditch effort to avert a Yishuv-Hashemite clash. Returning to Tel Aviv, she reported to the Mapai Central Committee: "We met [on 10 May] amicably. He is very worried and looks terrible. He did not deny that there had been talk and understanding between us about a desirable arrangement, namely that he would take the Arab part [of Palestine]. . . . "But 'Abdallah had said that he could now, on 10 May, only offer the Jews "autonomy" within an enlarged Hashemite kingdom. He added that while he was not interested in invading the areas allocated for Jewish statehood, the situation was volatile. But he voiced the hope that Jordan and the Yishuv would conclude a peace agreement once the dust had settled.(16)

On 15 May the Arab Legion, along with the other Arab armies, invaded Palestine. But far from unravelling, the agreement, at least in 'Abdallah's mind, appeared to hold fast. 'Abdallah's troops kept meticulously to the 17 November 1947 scenario: At no point in May, or thereafter, did the Arab Legion attack the Jewish state's territory. The Legion occupied the northern half of the West Bank and advanced as far westward as Latrun, Lydda, and Ramla (all UN-allocated Arab areas); and, acceding to local Arab pressures, drove into (Arab) East Jerusalem, essentially to secure the area (and its holy sites, including 'Abdallah's father's tomb on the Temple Mount) against Jewish conquest. But Jerusalem, allocated by the United Nations partition resolution neither to Jew nor to Arab, had not been covered in the Meir-'Abdallah agreement. Moreover, apart from securing the Old City (including conquering its Jewish Quarter), the Legion had refrained from attacking Jewish positions and areas (except for forays around the Mandelbaum Gate and the Notre Dame Monastery designed to safeguard the Legion's line of communication from Ramallah into the city).

No doubt, 'Abdallah was not motivated solely by his adherence to the agreement. His army was small, numbering only some 7,500-9,000 troops at the start of hostilities, and he hardly had enough soldiers to secure the West Bank, let alone attempt to conquer Jewish state territory or fight in costly street battles in West Jerusalem. Moreover, the British had warned him against attacking the Jewish state (see below), and he had promised to refrain from doing so.

Before 15 May, various Jewish officials - Yaacov Shimoni, Moshe Shertok (Sharett), etc. - feared that 'Abdallah might attack the Jewish state nonetheless ("Can any Arab be trusted?"). But in effect, when it came to brass tacks, 'Abdallah adhered strictly to the letter and spirit of the agreement. Rather, it was the Jews who broke it when the Haganah/IDF repeatedly attacked the Legion in Latrun (in late May and June), in Lydda-Ramla (in July), and near Tarqumiya (October). It was Ben-Gurion - because he sought to lift the siege of Jerusalem and expand Jewish territorial holdings beyond what the UN had earmarked - who violated it."

http://www.angeltowns.com/members/palestine/articles/Morris.htm