UV-visible absorption spectroscopy probes electronic transitions due to electronic excited states, where as absorption of IR radiation excites molecular vibrations and no electronic excited states. However, UV-visible radiation can also excite the molecular vibrations as well, and so what is observed is the superposition of the electronic absorption in addition to the vibrational absorption spectra. IR spectra are broadened by molecular rotations, which are caused by the absorption of lower energy microwave radiation (and rotational spectra can be extremely sharp).
If the species you are probing are atoms only, than they do not have any vibrations (because there are no bonds) and so the UV-visible spectra of atoms is very sharp.