1. Is there any reason ______ you can't come?
2. The reason _____ I'm late is because I missed the bus.
Which fits better in the blanks, why or that?
Is it possible that both fit?
In my opinions ,we will use " why" in both of those blanks.
I think so, too.
According to Understanding English Grammar by Martha Kolln (4rth Edition. MacMillan Publishing Company: New York. 1994. cited here), the relative adverb why modifies the noun reason and it is often omitted in that environment:
Ex: Do you know the reason why Isabel isn't in class today?
Ex: Do you know the reason Isabel isn't in class today?
Moreover, many writers prefer "that" in a clause referring to "reason":
Ex: I know the reason that men like motorcycles.
All the best.![]()