Bottom is the only "mortal" to interact with the fairies in A Midsummer Night's Dream. Why might this be so?

Bottom has a fanciful nature and Puck thinks he deserves a lesson because of his arrogance.

There are really two reasons that Bottom is the only one who interacts with the fairies. One is that Bottom is already a fellow with a big imagination. The other is that he is supremely arrogant, in a clumsy sort of way. In other words, he is perfect as the butt of Puck’s joke.

Puck sees the craftsmen practicing...

Bottom has a fanciful nature and Puck thinks he deserves a lesson because of his arrogance.

There are really two reasons that Bottom is the only one who interacts with the fairies. One is that Bottom is already a fellow with a big imagination. The other is that he is supremely arrogant, in a clumsy sort of way. In other words, he is perfect as the butt of Puck’s joke.

Puck sees the craftsmen practicing their play and decides that they are too close to the fairies. He observes them and notices Bottom’s antics. From the beginning, he wants to play every part, he overacts, and he is generally full of himself.

I will move storms, I will condole in some
measure. To the rest: yet my chief humour is for a
tyrant: I could play Ercles rarely, or a part to
tear a cat in, to make all split. (Act 1, Scene 2) 

All of this is happening in the middle of a war between Titania and Oberon, the queen and king of the fairies.  Oberon instructs Puck to help him get revenge on Titania, who is causing trouble because of her continued possession of a changeling they both seem to want.

Puck decides to have some fun with Bottom and Titania.  First, he exchanges Bottom’s head for that of an ass (a donkey). This terrifies the rest of the craftsmen. Then he uses his love potion to make Titania fall in love with Bottom. The fairies all have fun with Bottom until Oberon decides enough is enough and reveals his trick. 

OBERON

There lies your love.

TITANIA

How came these things to pass?
O, how mine eyes do loathe his visage now! (Act 4, Scene 1)

The trick is all the better because it was played on Bottom, the arrogant and silly weaver. Bottom could see all of the fairies when he was with Titania, but he was just conceited enough to go along with it. Once Titania came out of her spell, the fairies had no need for Bottom anymore.