Hi
Working on some training material for controllers. I have seen several reasons for this restriction.

Can anyone tell me the exact reason why this restriction applies.

Thanks

A possible reason is that there are more planes below FL10 so if they are moving slower they are easier to manage? I have no licesense or training but this is just my thought on the matter, sorry if it shouldnt be here

From 'Search'

The comments by Groundloop in the alternative thread offered by BOAC are correct. The 250kt restriction was a direct recommendation from US authorities as a result of the New York midair in 1960 and was subsequently picked up and recommended by ICAO for immediate international implementation by all member states

And QSK? Is right when saying that Groundloop is correct when referring to US rules.
FAA’s philosophy was to give sufficient turning radius to allow enough distance for IFR and VRF avoidance. VFR traffic must keep a greater distance from clouds and have better visibility above 10.000 because IFR traffic may “bump” out of clouds at a speed higher than 250 KIAS above 10.000 feet. VFR weather minima below 10.000 feet was made under the assumption that IFR traffic hidden in clouds would operate at or below 250 KIAS. (Ref US FAR § 91.155 Basic VFR weather minimums)