Some argue that the number be frozen. Details here. It may surprise some to note that the UK's House of Lords does not have any set number of seats. Yes, that's right. The number of Lords is not fixed. This matters because new governments often agree to a majority of new Lords from their party. The House of Lords must normally agree legislation with the House of Commons for it to become law.
So why is this is the case? As I've learned about many things British, you shouldn't ask "why?" but "for how long?"