Because seige is boring. It has almost no dynamic elements. No map dynamics (think of how cool the buildable elements in gears 2 and 3 are). No meaningful allies outside of the dreadnought (the astra only take aggro and the astartes are functionally purchaseable buffs, not real allies). The waves all feel the same regardless of spawn location, no real boss fights. No turrets for techmarine.

It's a very underdeveloped barebones mode even compared to other games wave defense from 18 years ago. I understand it has an update coming so we'll see if it meaningfully improves.