The law for this is in the Immigration Rules .....
click here. In particular read para 276B(i)(b) where it mentions 14 years.
What I am just about to write may not be the full history, but I can say those provisions in the Immigration Rules were only added a few years ago. Before that there was no legal provision of Long Residence applications for ILR. Instead there was a Long Residence Concession, not officially part of the Immigration Rules. For years the Government of the day preferred it that way, agreeing to some Long Residence applications for ILR, but giving people no rights to insist upon their applications being looked at properly.
So it was a good step forward when the provisions were actually added to the Immigration Rules, a few years back.
I suggest you do a Google (or similar) search on .... Long Residence Concession .... for more on the history. For example I have just found
this link which mentions the date of April 2003.
If you do find the answer to the question, why 14 years, please do post.
Edit : This might be rather helpful ....
click here .... for the history of the concession.
