(Not sure if the humour went over my head and I’m explaining something that wasn’t meant to be taken seriously. If it is humour, then I imagine it’s because knowing what the H stands for is a lot less exciting than knowing what the p stands for. If it isn’t humour, you may read the following words.)

Because that’s what it measures. It measures the amount of H⁺ ions (which are simply protons if we are talking about protium which is the most common hydrogen isotope) by volume on a logarithmic scale.