Because running a stadium costs more the bigger it is. You have to man it, stock it, light it, heat it, pay for the extra however many millions of tonnes worth of building materials etc etc. If they don't think they can fill a 80,000 then it's more effective to build a 50,000 seater. Let it make you money and then use that money to invest in the playing and coaching staff. Over time as the club becomes more successful we expand the stadium and our business model and it means we don't have to rely on a sugar daddy but instead moshiri has taught us to fish rather then just giving us a fish