Larger machines cost alot more $$, they require larger motors, thicker, stronger materials for the frame and parts, and expensive linear motion systems. They also take longer to build.
They take up alot of space, so if you only have a small shop or basement then a large machine is out of the question.
I am actually in the design phase of my new machine which will be a 5x10 cutting area, I expect it to cost me around $15-20,000 and I plan on taking about 5-6 months to develop/design it and another 5-6 months to build it.
If you have the time, budget and space then a large machine can be made, but most of us here are short on one of those 3 things, so a compromise is made and a smaller machine results.