The reactor temperature does not continue to increase uncontrollably because as the conversion increases, the concentration of the reactant decreases.  Observe the rate expression.

In this rate expression, the rate constant is a function of temperature, and the reaction rate is a function of the rate constant and the concentration of A.  As the rate and the temperature increase, the concentration of species A decreases.  This a situation where there are two competing effects.   The final outcome is a steady state which is achieved when these effects balance each other.  For more details, see Appendix C.