Hey there ! I think that I have an alternative answer to this question .
The man's wife is waiting for him to get home.However, he is having an affair with a lady on the 7th floor and visits her before he gets back after work.When it is raining , he goes home because his wife would notice that he is dry or his umbrella is dry when he gets back into the house , hence suspecting him of being in the building for a long time before he steps into the house . If there are people in the lift , they would see that he is going to a floor which he is not supposed to be on ,raising suspicion. So the only time he can go is during a non-rainy day when there is noone looking at him . What do you think?