Comparing the experimental and calculated UV/vis spectra for ethene
In short, there are two obvious problems with the setup OP uses for TD-DFT calculations:


B3LYP functional is not a good choice for TD-DFT.
6-31G(d) basis is usually too small.


At M06-2X/Def2-TZVP level I get a maximum at ~160 nm, which, taking the accuracy of the TD-DFT approximations into account, is close enough to the experimental value.