Flint is supposed to use fire type Pokemon, but the only fire type Pokemon he has is Infernape and Rapidash.The rest are Steelix,Lopunny and Drifblim.Which are : Steel-Ground,Normal and Flying-ghost. so why ?
That was how gamefreak wanted it but it was only for DP games.Also you should notice that all of his Pokemon know some fire type moves like lopunny knowing fire punch,steelix fire fang and drifblim will-o wisp.IN platinum,all of his Pokemon are fire types.The new fire types were houndoom,flareon and magmortor.Source