The Balfour Declaration is a torturously worded document that went through extensive revisions between its initial proposal and its final publication. Of the major Middle East post-war promises, the Balfour Declaration is the only one that was made publicly. The other agreements that Britain sought to reconcile after the war included the Hussein-McMahon correspondences (to the Arabs) Sykes-Picot (to the French) and their own self-determined war aims as outlined by the DeBunsen commission, not to mention a number of later strong implications that Abdullah (of Transjordan) would eventually rule Palestine as well upon British withdrawal.

As Balfour was negotiated last, it's relatively easy to spot how lawyerly they were being:

His Majesty's government view with favour the establishment in Palestine of a national home for the Jewish people, and will use their best endeavours to facilitate the achievement of this object, it being clearly understood that nothing shall be done which may prejudice the civil and religious rights of existing non-Jewish communities in Palestine, or the rights and political status enjoyed by Jews in any other country.

Notice the use of terms such as "view with favour" rather than, say, 'guarantee', and the essentially meaningless "national home." Particularly "a national home [in Palestine]." The point being that nowhere here is the Balfour declaration promising a Judenstaat or an Israel.

As for why British policy in the interwar period took the shape that it did, the greatest pause amongst British policy makers was in trying to ensure "that nothing shall be done which may prejudice the civil and religious rights of existing non-Jewish communities in Palestine". As it transpired, protecting these rights and carrying out "the establishment in Palestine of a national home for the Jewish people" proved to be contradictory aims. The result was vacillation in British policy between Balfour in 1917, the Churchill white paper's immigration restrictions in 1922, the Peel commission's partition proposal in 1936, the 1939 white paper's immigration restrictions, and eventually the abstention of the UK from the UN partition plan vote in 1947.