Question: 

Why Tube-To-Tubesheet joint check and Axial tube stress are not calculated for U-tube HX?

Answer: 

UW-20 requirements are not applicable for U-Type construction as per code.

U-tube cannot have their welds checked with UW-20 because in a U-tube there are no axial loads. There is no Ft. Welds cannot be full nor partial because there is no axial load. Ft is only calculated for non U-tube tubesheets.

See below code reference,