Bird strike is only part of it.
Then main reason for 250 max below 10,000 ft is because that is what I call indian territory... That is where you will find the indians.. Cherokees, along with Cessnas and all manner of slow moving aircraft..
Also the cloud clearance and visibility requirements for VFR aircraft are based on a max airspeed of 250 KIAS.. Above 10,000 ft the VFR visibility and cloud distance requirements are greater due to the higher speed.
Bird strike requirements should be met by a combination of operating limitations and procedures such as Vmo and Mmo limits.
Bill