Why? Mostly because it is pointless, or at least their label is.
Simple, life (sentence) in the UK does not mean a person will be in jail for the rest of their days (unless they manage to get out by a merciful act of state, or by being found innocent). Currently, a "life" sentence is meted out with the caveat "to serve no less than N years". And you'd think it means that a person is not eligible for parole before serving those "N" years. Sadly, that's not what happens in reality.
In reality, the "life" sentence described above really is a sentence to "N" years in prison. I am yet to learn of a person who wasn't released from their "life" sentence early (i.e. not in a coffin).
Now, you know I am no fan of capital punishment. I've written about it here, too. But for some crimes, in absence of capital punishment, the only correct sentence is life, no parole. Ever. Complete removal of an individual from human society, mostly for the protection of latter. With some "life" sentences in the UK recently having a really laughable "minimum term" (isn't minimum term for life, death?) I wonder if robbery may not be a worse crime.
So, where I put my voice squarely behind "abolish capital punishment", I also put it very squarely behind the call for "life means life".