It's not really due to them being cheaper. Topre TKL are not cheaper than full-sized ones. FILCO's are just about the same price.
It's just about a fact that, you want more space on your desk. You want a cleaner keyboard, a more usable one, smaller with less unnecessary parts.
Think with me. The distance your right hand travels between your mouse and the keys themselves gets shorter when you don't have the numpad, right? The mouse gets closer to your keyboard and you can switch faster between typing and using the mouse and the fatigue over time also gets reduced.
And, well... Many people just don't use the numeric pad, so why not tossing it away then?
Many, many, many people in here use their keyboards as work tools on offices. That's why they dislike macro keys and their love for TKL keyboards is so great. They just don't want a keyboard that wastes space on their desktops, they want something usable that won't be a bother when their desk is full of papers and other materials.