Why is Vt usually 26 mV in the diode formula?
K is Boltzmann's constant, which is (ahem) constant. 

q is the charge of an electron. Similarly constant. 

T is temperature. It's not constant, but humans exist in a fairly narrow range of temperatures around 300K. So 25 or 26mV is a reasonable value for room temperature Vt.  

By the way, the 't' in Vt stands for 'thermal' so that should remind you that it is a function of (absolute) temperature.