Why am I not getting maximum deck acceleration at mid span for a simply supported condition by application of dynamic nodal loads using Time History function?

Actually, the behavior of the structure under dynamic load depends on many parameters, such as the load magnitude, loading interval, damping in structure, etc.
For such a high-speed train, the accelerations is varying much more in different parts of the structure.
The behaviors as you are expecting might be available at low speeds, but not at high speeds. This behavior for high speed load is acceptable. On applying higher damping, it can even be noted that the acceleration is higher towards the end of the bridge, which is how it should be.