it could be money issues.
ie if you need fast cash quick, you sell stuff and your tanks end up on the list.
I don't think its general cost of maintaining them since fish are relatively cheap, so is fish food and water and hydro are nothing compared to the hydro and water most people use in daily living (showers, baths, dishes, heating the house etc)
the startup costs of a big setup are expensive (tank, stand, filters, lights etc)
after that, its relatively cheap.