Flak wasnt as useful, because there were means of disabling the radars that ranged the flak, the guns often werent concentrated enough (with many notable exceptions, but you couldnt cover all of germany with guns, but you could with planes moving at 100s of miles per hour). Flak, mainly the 88 was a good weapon and certainly could reach up and strike the formations of heavys, but B-17s and B-24s could take alot of damage, and flak is just small bits of metal moving at high-speeds due to an explosive 88mm in this case shell. It would take a direct hit on the aircraft, the death of pilot and co-pilot, or severe flap or engine damage to bring down a bomber of the size of the heavies. Also, in such a large formation, the odds you will hit the same bomber more than once are slim, making repeated near miss bring downs much less likely, they needed fighters and flak coordinated for maximum effectiveness.