Because they copied the French, literally. During the First World War, it was necessary to distinguish Allied planes from their German adversaries to avoid friendly ground fire. The British experimented with a number of designs, the the Union Jack among others. The problem with these designs that from a distance,!it was difficult for observers to discern the St George Cross segment of the Union Jack from some German symbols such as the Iron Cross. Due to these issues, the British followed the French lead, introducing a roundel in the style of the tricolour cockade. To distinguish British aircraft from French, the British reversed the colours, replacing the red outer circle with the blue and the the blue dot with a red.

I don't know why the RAF retained the colour scheme after the war rather then replacing it with one more reflective of their style but I imagine when it became clear that the RAF would be operating alongside the French at the outset of the Second World War, it was thought a good idea to continue the use of the RAF roundel so both British and French ground forces could easily distinguish planes as Allies.

Edit: Corrected mistake concerning the St George Cross