I'm afraid I don't have much of an answer to the historical question (just speculation, nothing sourced), except to say that the chi-rho is not completely out of use by Christians, though of course the cross is much more common. But as for a theological question:

It seems like a better symbol to represent the religion than the cross as it doesn't glorify the crucifying of Christ.

Christians have every reason to glorify the crucifying of Christ, for the importance that this holds in the whole story of salvation, and the necessity of Christ's atoning death to open the possibility of their salvation.