Because if you have a minimum wage 40h/week job and it goes any higher than the 20% it is at now (it was 10% only a year a go), it gets really hard to pay the rent and get enough food to live. I'd rather have higher VAT on things that are 'luxury' than taxed on my wage that pays for things I *need*.
But, as has been pointed out, the poor lose proportionally more on VAT rises than income tax rises. This isn't a debating point, it's an observation of all current and previous changes in VAT levels. And, as has been pointed out repeatedly, you're not really talking "luxuries" here, you're talking about the things people typically buy in life - they may not be strict necessities, but there's a big excluded middle between necessary and luxury.
(Plus the rise in personal allowance will help balance the tax system anyway which should help the working poor anyway.)