Why do you say it's worse? The HV is, in it's way superior. It can actually destroy ships by itself, with no fleet needed to complete the job. The Ion cannon, while it completely disables ships for a time, will never be able to destroy them outright. Depending on the fleet you have above the planet, they both have their benefits and drawbacks.
Because the HV costs a lot more, requires two build slots to have the same timing as the Ion Cannon, and takes a lot longer to build, in addition I consider the ion cannons better for not only that reason, but also for making heroes and SSDs a cakewalk.