Well let’s see …
- The Tomahawk has 6x the range of the Brahmos, a 50% larger warhead, and is half the size, but only 1/3 the speed. The extra speed is not worth much if you are dead before you get in range.
- Tomahawks use the standard VLS launch system and can be swapped with SM-2, SM-3, and ESSM quad packs as needed. Brahmos requires a dedicated launch system which eats up a lot of scarce deck space. Typically a ship with Tomahawk missiles will have 3x to 5x as many as one equipped with Brahmos missiles, making it much more likely to saturate the target’s defenses.
- Brahmos had a larger radar signature than Tomahawk, and a huge thermal signature, much easier to spot. The US Navy has been practicing shooting down missiles like the Brahmos for 30 years, they don’t seem very worried. Something like 1000 Tomahawk cruise missiles have been launched in anger, how many have been shot down?
Having the speed of the Brahmos is nice in the anti-ship role, but it is hardly a panacea. In the land attack role, except over extremely flat terrain, high speed and low altitude don’t mix well, forcing the Brahmos to fly higher than the Tomahawk and therefore more vulnerable to detection and engagement. Combined with their much longer range allowing the approach to take place from unanticipated directions. Tomahawks have frequently been able to take advantage of blind spots to sneak into very close range of the target before being spotted, Brahmos cannot do this.
Doesn’t mean that the Tomahawk is always better, or will always be good enough. The US is working on both stealthier subsonic and hypersonic systems when they are needed. Odds are that these systems will be the same size as the Tomahawk, but with less range or smaller warheads. Switching to something the size of Brahmos would require redesigning most of the ships in fleet.