Because it would not work. At all. If you must have an upper chamber, then in order for it to work it either has to be a weak revising chamber or a powerful body in its own right (the latter being a very bad idea of course, as the US Senate shows). Clegg's proposals manage to combine aspects of both in a way that's almost impressive in its lack of logic. It would also make certain specific problems that the Lords has worse; you are not going to solve the problem of lobbying by creating posts elected for fifteen year terms! There's a massive corruption scandal - and
then a constitutional crisis - just waiting to happen right there.
Or, for that matter, patronage. Who will control the rights to these precious fifteen year terms, exactly? The party leaders.