Who says the church members or the church building are The Kingdom of God? Christians don't believe that.
The Gospel of Thomas says the Kingdom is in and around the people.
As for the other part of your question…
A lot of Catholics from my parents’ and grandparents’ generations believed that you “had” to go to church every Sunday because if you didn’t you were a heathen and would burn in Hell.
They were “scared” into thinking that they needed to attend church and being fed not just God’s word, but also the political messages the church ingrained in them.
And Ozmo is right about the Catholic church having a problem with it.
I believe the reason the Catholic church (among others) refuses to recognize Thomas’ Gospel is it downplays the authority and importance of the church.