Please explain to me why when the word 'li' is placed before a word that begins with an 'al', the 'alif' dissappears (its not even written with a vowel above it, it is completely removed).

Question
'Li' before the definite article 'Al' (The)

Please explain to me why when the word 'li' is placed before a word that begins with an 'al', the 'alif' dissappears (its not even written with a vowel above it, it is completely removed).

eg. li + Allah = lillah

Jazakallahu khayran.
Thank you.

Answer
When the "jar" or with a kasrah precedes a noun starting with hamzah al-wasl, the hamzah wasl (written on an alif) is removed in writing and we do not use hamzah al-wasl when starting this word because there is an added and we have no need to use hamzah al-wasl.  This is a spelling rule specifically for the hamzah wasl when preceded by this in a noun, and this rule is that the hamzah wasl is removed in writing and not used when starting the word. 
Please see the following link:
http://www.abouttajweed.com/hamzah-al-wasl/hamzah-al-wasl-lesson-6.html

Wa iyyaakum.
You are most welcome