According to Rashi, Lot was being given a chance of survival that was denied to the other inhabitants of the doomed cities, even though he had sinned like they had. It was not right that he should witness their destruction.
Rashbam adds that looking back might remind him that his two sons-in-law were trapped in the cities, and this might deflect his attention from his own and the rest of his family’s survival.
In a broader context it is a good rule that a person should not look back at what is now past.
There is a temptation to say, “I know I have moved to another country, but I am having my regrets”… or “I know I have retired, but now I’m feeling sorry I did”… or “I know I have sold the house, but I wish I hadn’t”.
Something can sometimes be salvaged from the past, but as a general principle one has to move ahead and build a new future.