Because you're pushing less weight when you use the wall. Do as much as you can against the floor and then move to the wall. You'll benefit from those wall ones and they will help you gain more reps against the floor over time.
As for me, I do 4 sets against the wall after I'm done with elevated feet pushups. However, I split this into two different sets. One set of two I will push with my fingertips as if I'm grabbing a ball. I feel that this works the top muscles of my forearms much more, which is an area that I lack. The next two sets, I will create a diamond with my index and thumbs connected as I feel it concentrated more on the chest muscles by this point.
Eventually, you'll be able to elevate your feet, which will target your upper chest and make the movement much harder. Then, you can move into a normal pushup position, and then finish up against the wall.