Why is the drogue parachute jettisoned on the runway?
For several reasons.  


First to protect the drogue from damage by scraping on the pavement
second to prevent it from becoming snagged on nearby objects
third to prevent metal components on the drogue from hitting and damaging the tail of the aircraft,
and fourth to make it easier to taxi back to the ramp without the need for higher power setting to overcome the drag and prevent damage to the drogue from this jet blast.


One downside to jettisoning the drogue at a lower speed (&lt; 40 kts) is that it now creates a FOD hazard on the runway and requires a waiting cleanup crew to retrieve it before another recovery is possible.