Galileo used inclined planes to investigate "freefall."

Why did he do that instead of experimenting with velocitydropped

objects?

Because of the reason that the rate at which the velocity dropped object fall depends on their mass and may vary.

Explanation:

Galileo's investigation of free fall of objects using inclined planes led him to measure acceleration accurately and to conclude that all objects fall at the same rate irrespective of their masses under the action of gravity when the external forces on the object are absent.

Whereas as far as the velocity dropped objects are concerned they acquire different velocities for variation in masses.

Thus the velocity dropped objects may drop or fall at different rates and not at the same rate.