这个属于固定用法,就像一些phrases 和collocation一样,讲不出具体原因的.
固定句型是：This is the ……time that sb.have/has done sth.
This was the ……time that sb.had done sth.
e.g.:This is the first time that our whole class have gone out together for a picnic.
He said that this was his third time that he had been to New York.