A few random reasons off the top of my head
a) we see Fir Park as being an integral part of the club's history and spent many happy years there
b) we have seen the lego/flatpack stadiums built elsewhere and think they are crap
c) we know that selling the land Fir Park is built on is likely to leave us a few million short of funding a new ground
Have you seen a business plan, because right now I don't see where the financial security would come from. Only more debt. Once I see the new stadium plans, and the financial projections, I might start supporting it. Right now, all I can see is a crap ground with tiny plastic seats, no elbow room and, possibly, plastic grass.