From what I've been told, they can't do much with Chocobo Racing because some of its coding is tangled up with ARR weather and modifying the racing could mess something up in the overworld, there's also code that ensures the game keeps your racing chocobo and companion chocobo separate, and one wrong move and they get fused somehow. To top it off, the person who was on the team to do the programming for chocobo racing doesn't even work on the game anymore, so they would need somebody else to go in to untangle the mess.

Not to say Square-Enix shouldn't do SOMETHING because Chocobo Racing is a more interesting side activity than most, but I can see why they ignore it in favor of adding/fixing other things because it was never super-popular, and they would need to tread carefully going through the spaghetti code otherwise they'll break three different things.