While calculating concentrations is trivial (divide the amount of substance by the volume), activity is a complete different story. Activity is usually calculated by multiplying the concentration by a dimensionless number called activity coefficient($\gamma$):
$$a = c \times \gamma$$
The problem here is that $\gamma$ is not only different for every substance, it also varies with temperature, concentration of the substance itself, the diluent used, and so on. If the substance is a salt that dissociates then the calculation gets even more complex.
Since having the values of the activities would be quite a challenge, equilibrium constants are often written using concentrations, which are more readily available. It's not the most correct way of represent it, but it makes your life easier.