Given all the positive characteristics of venus, why has space exploration focused so much on mars?

What "positive characteristics" of Venus are given ?  If you mean a planet habitable by humans, Venus has temperatures to cook meat, plus a thick, soupy, acid atmosphere, and conditions on the surface capable of crushing, dissolving, and melting a battleship.  Mars has very little atmosphere, and is cold.  If you dress warm and carry a bottle of air with you, them you've got a chance on Mars.

when two balls of the different masses are dropped from equal height, they reach the ground at the same time .this is because they experience the same acceleration.

the acceleration is independent of mass.

thus when they are dropped they will always maintain the same velocity and travel the same distance (neglect air resistance of course) at the same time.