Why my brake pedal on my ford expedition 99 model goes to the floor with new brake pads on?

Why my brake pedal on my ford expedition 99 model goes to the floor with new brake pads on?

The other answers are true. Except when you have ABS there's an electronic valve setup that must get cycled to release the trapped air. That is, if you introduced any. Dear.
You can purge the mastercyl at the outlets, the calipers at the wheels, make sure you haven't left any bleeders unbled throughout the system. Except when you regain a little bit of pedal, you have to operate the system. Then rebleed.
At the dealership we use an electronic bleeding setup which actuates the valves. Then we bleed it. Actuate it, then rebleed it.
while its still on the lift.
If you go to drive it be very careful and be ready to use the parking brake. Operate it to run the pump and actuate the valves. Then immediately begin the bleeding sequence.

Improper bleeding I would guess.

Did you pump the pedal long enough?
If during installation you pushed the pistons back into the cylinders to fit the new pads, then there may be a fair bit of a gap between the pad's surface an the brake disk.
The first few times you push the pedal there will be almost zero resistance, because the disks only move a little closer, reducing the gap.
The pedal starts feeling firm again when they actually press hard against the disks.

Other possibility is that air got into your system. E.g. If the brake -hoses or -tubes got loosened or the fluid level in the master cylinder was allowed to get too low.

Yep, air in the system. It hasn't been bled properly.