because alot of people dont own their home and moving a 1000gal tank is almost not worth it
selling a big tank is not always easy (friend sold a 400gal glass tank 6x 1st 5 people showed up in a small truck with 1 helper and backed out when they had to hire movers)
big tanks are expensive to buy and maintain
maintance on big tanks sucks they need to be setup as close to maintance free as possible
alot of people get obsessed with huge tanks (no wonder most of us are men) when if they spent a little more time and thought on making what they have better they would be happier just my own expierence
when 1st started keeping fish i wanted more fish that needed more room if fish spawned i tried to keep every fry
when i do finally buy my house hoping i dont move again i had a wall length built in tank in mind so i would say 500-700 gal area with a sump and filtration room in the basement ..