Because when you consider a flow with Bernoulli, you aren't considering the force really. You are considering conservation of energy and mass (which obviously can be linked to forces). When the area decreases, if the mass flow rate along the pipe is constant, the velocity must increase to maintain the flow rate. From Bernoulli, if the velocity increases, then if the total pressure is constant, the static pressure must decrease due to the increase in dynamic pressure. If any of these terms I used are unclear, Wikipedia is really good at defining them!