No logical would be saying that: Both focus on day to day life and peoples relationships; Air is slightly slower paced and more of a romance drama than Haruhi but the similarities are enough that if you liked Haruhi you may enjoy watching Air.
Hence I couldn't go into much detail other than the day to day life and relationships thing. The two really aren't that similar but atleast my explanation has some relevant basis for them being likeable by the same audience.