"Why not?" is sometimes a question that can't have a clear answer. I can just help give a little bit of ideological context because there is a little backpedaling to do there to avoid applying present thinking to a time in history when the thinking was different.

The idea of a Palestinian state is something that is born out of Palestinian nationalism. Nationalism, the self-determination of nations, and the Palestinian national identity were all (warning: controversy) relatively nascent ideas in the region at the time. While Europe had just wrapped up the most recent and most terrible wars driven by the emergence of Nationalism (among other things) in the Arab world, the ideas behind nationalism, the primacy of national identity as a political identity, and even what the national divisions were was not established (regional vs. pan-Arab nationalism). The idea that the Palestinian people should have their own state would not have been common.

Some would argue that the shared Palestinian national identity was essentially born out of the tragedies that befell them in 1948 and after, and the experience of exile. Palestine had not been a sovereign territory before the British took control of it with the fall of the Ottomans. When the British left, no prior state had existed in the area to lay claim to the lands held by Egypt and Jordan after 1948. Egypt was more on board with Palestinian self-determination than Jordan at the time. Egypt only held the Gaza strip in a military occupation for a time and the Arab League established the All-Palestine Government, which was largely ineffective but was meant to lead to the birth of a Palestinian State. King Hussein of Jordan wanted Palestine to be part of his kingdom and opposed any Palestinian self-determination, preventing the All-Palestine Government from holding authority over the West Bank.

I'm by no means an expert, but since no other substantive answer has been given yet, I'll bring up a few points.

1.) The designation of the West Bank and Gaza Strip as the, "occupied Palestinian territories" wasn't made until after the 1967 war. Prior to this, the international community considered these borders to be Armistice Lines, areas where the Israeli and Arab armies finally stopped fighting in the 1948 war.

These areas included large tracts of land reserved for the Palestinian side of the UN partition, that were annexed by Israel during the 1948 war. The international community did not officially recognize this land as part of Israel (yet), so they had no reason to call for a Palestinian state on the West Bank and Gaza.

2.) Jordan was not interested in the formation of an independent Palestinian state. King Abdullah of Jordan had struck a deal with Israel in the run-up to the 1948 war, where Jordan would annex the West Bank unopposed by Israel, while Israel would defend and expand its borders unopposed by Jordan. After the war, Jordan attempted to fully annex/integrate the West Bank into their country, (while Egypt at least was somewhat interested in maintaining the Palestinian identity of Gaza).

3.) The broader Arab and Muslim world was still set on reclaiming all of mandatory Palestine (Palestine + modern-day Israel). The Arab armies had been defeated in 1948, but there wasn't yet a feeling that Israel was militarily superior, and so hope remained that Israel would one-day (soon) be defeated.

This outlook would change after the 1967 war, where Israel would decimate the armies of Jordan, Egypt, and Syria in a matter of days, and occupy the Sinai, Golan-Heights, and West Bank, all with minimal effort.

The desire for "historical" Palestine remained, but the Arab world began to realize that Israel couldn't be defeated through strength of arms, and soon acquiesced to the international call for Palestine on the West Bank and Gaza.

Hope this helped.