New laws require legislative acts, which are hard enough to get for uncontroversial things that a majority of people want.

The activists took the legal approach because there was potential to get the goal they wanted without legislation. A judge could have theoretically ruled that Happy constitutes a legal person, and was thus entitled to the protections afforded to legal persons. They didn't, but they could have.

I suspect that if not the activists involved in this example, some activists are working on getting legislation passed, but this probably seemed like a tactic worth trying as well.