Why didn't Congress respond with troops when the governor asked for assistance during Shays' Rebellion?

It was basically due to two reasons firstly,

They didn't have power over the states to send in an army

Secondly,

They didn't have money to fund an army at that period.

Explanation:

At that period, the congress did not have any power to taxation as it was all in the hands of the state's to collect taxes. Due to the inability of the congress to collect taxes, they were unable to gather enough money to fund an army and as such when the Governor requested for assistance, they could not oblige.

Also they had no power to send in military forces.