Why didn't Lot (peace be upon him) call his people to seek forgiveness and repent, like other prophets?

The Prophet of Allah, Lot, called his people to the fear of Allah (taqwa), which includes performing all commanded acts and abstaining from all forbidden ones. His call focused on forbidding them from the gravest abomination, which was men lustfully approaching other men instead of women. This prohibition was a command for them to repent and return to Allah, and it does not necessitate the explicit utterance of the word "istighfar" (seeking forgiveness).