I'll answer: because it can be launched from places the enemy cannot touch, while Brahmos needs to get up close and personal at a relatively close 300 km which is well within strike range of carrier based ships.
Let me put it another way: China can sink a US carrier at dock in Japan, from a missile launched in Heilongjiang. The Brahmos, however, to sink a US carrier in Japan, actually has to fly close to Japan, or wait for the carrier to move if that's not possible.
Due to this huge range difference, they are not comparable. They're used in different ways. Staggered DF-21 can create a very, very deadly, one sided shooting field in a 2000 km radius around Chinese shores where no one can fire back against DF-21 mobile launchers, but DF-21s can just keep shooting. Brahmos has to be carried into battle face to face, where it is not one sided.
well mate i would like to tell u that brahmos 2 range can be increased with ease ,but due to mtcr it is restricted to 290 km ,but it could easily have 700-800 km range & i hope india would be doing it in case of war .
while brahmos 2 has flexibilty of launch platforms
u r only pointing out stealth ,ok i would explain all ur points
1 .Subs : see todays AIP enabled SSk & nuke subs are very quite & deadly & Sub launched ASCM is the deadliest threat to any ship ask any proffesional they would also say the same thing .
2.Ships : these are also becoming stealthy but i assume their role would be for destroying surface warships but not carriers.But todays ships also have advanced air defences
3.Fighters : same thing 5th gen fighters are stealthy ,so out many fighters assign to destroy a carrier 1 can surely penetrate any layered defence of carrier group for sure
About range of DF 21d see more the range it also has advantages & disadvantages .
Adv : it can destroy it's target from long range .
Disadv :1.u must have accurate guidance for BM ,cep value must be less than 25 i assume to hit a big carrier also the target is not static it is
mobile
2.it can also give more reaction time for enemy ship to deploy it's air defences.& also it has more chances of being detected .
Also i had posted it's counter weapons in 1st page of this thread kindly read it .Most probably it can be intercepted though SM3, but it can be effective if they deploy MIRVS or overwhelm the enemy's airdefences by firing atleast 20-30 ASBM at single time for a single aircraft carrier but it would be costly
Well it can only target AC while it cannot target small warships like FAST attack vessels ,while Brahmos 2 can target anything at such short range with pinpointed accuracy