There are basically two reasons offered.

The first is a sense of "owing" them the help, because of the special status of the Jewish people during the Holocaust, and many French scientists and statesmen feeling culpable.

The second is a more strategic reason: a belief that France's interests would be well-served if the Arab nations were further wrapped up in difficulties in the Middle East, as opposed to Algeria. So the idea here is that this would complicate and occupy Egypt-Israel relations to such a degree that Egypt would find it difficult to spend much time worrying about Algeria, thus making life easier for France.

Both of these motivations are present in the French archives; it's unclear to me which was dominant. For scholarly discussion of this, see Kroenig, Exporting the Bomb, chapter 3.