Walking on the ground is possible due to friction. As we step forward using one of our feet, then the ground surface and the foot surface gets interlocked between each other's minute roughness, as a result of which that particular foot gets a support to let the other foot come forward, thus making the walking process more easier.
The layer of soap makes the floor smooth due to which the friction is reduced. This means that the soap acts as a lubricant when spilled on the floor. As the floor has some minute roughness which making the walking process easier, but due to the presence of soap, it fills the roughness, thus making the floor smooth and slippery.
So when a person tries to walk on such a floor then he/she loses grip or balance due to lack of support on any foot which is a result of less interlocking or even no interlocking. Therefore it is difficult to walk on a soapy floor.