Three possibilities
I would say that the longer they are and the closer to the edge of the tool they sit, the easier it is for them to become dislodged & damaged. Thus, they would require greater fixing and this would add considerably to the cost.
Also, the pressure applied from the user to the floor tool is exerted entirely to the middle of the tool, therefore this is where any intense activity needs to take place as it makes full use of the pressure available.
Add also to this, if the thread pickers sit around the main suction hole, anything which they loosen will be sucked straight up.
I have to say though, I never thought the thread pickers made much difference to the cleaning performance. Time was I sold a universal tool which had a huge velour pad, a lot like a clothes brush. They worked very well on pet hair, cleaning hair off on one stroke, and releasing it into the suction hole on the other. The pad fitted either way round, so the user could choose whether to collect hair on the back stroke and release going forward, or vice-verso.
Another variety was bigger and sported a suction control valve. This was more suited to carpets than furniture. It had two long pads on either side of the suction hole. They had the velour pile in opposing directions, meaning one pad collected hair whilst the other released it on alternating strokes.
