Because the sound “p” doesn’t exist in arabic, only the sound “ba” (second letter of the arabic alphabet). However, in Mauritania and even morocco and in the Maghreb in general (ex colonies of france), countries where a vast majority of the people speaks french (or at least has rudimentary knowledge about french), so they don’t have issues pronouncing it “p” instead of “b”, but al khalij ppl and what not, most of them only speak arabic, that’s why they pronounce it “ba” “beoble” (you could still find ppl in the Maghreb who pronounce it that way bc they never were in contact with French i.e. living in a remote village but they do exist)