Why is alif not considered a leen letter despite the fact that it is preceeded by a fatha and is sakin?

Question
i have a question, im a bit confused- why is alif not considered a leen letter despite the fact that it is preceded by a fatha and is sakin?
Answer
The alif is a medd letter, and it is preceded by a fathah as it root structure.  There is no alif if not preceded by the fathah.  The alif is lengthened two vowel counts when not followed by a hamzah or a sukoon.
The leen letters, which are the saakinah preceded by a fathah, and the saakinah preceded by a fathah are not medd letters.  Instead if there is no sukoon (temporary or otherwise) after the two leen letters, there is a lengthening of less than two counts.  It is longer than a harakah but short than a medd letters. 
Without the lengthening of two vowel counts for the alif, it would not be an alif, so it cannot be a leen letter, since it is inherently lengthened two vowel counts. 
Insha' Allah this clears things up for you.