Because vouchers treat people like schoolchildren instead of adults capable of making sound decisions on their own. Having to claim benefits is bad enough without being infantalised as well. And vouchers don't account for odd expenses/items you don't normally have to pay
And believe it or not, just because you're going through a hard time, doesn't mean you have to do without any treats and nice things at all. That's really stupid
I think I'm perhaps one of two single mothers on this thread (along with DameKew) who actually fully put myself in this position. But for the rest, it takes a sperm as well as an egg to make a baby, in case you need to revise some basic biological facts. Oh no wait, fathers are blameless and his problems are all her fault 
And yes, I've claimed benefits as a mother. And spent some of it on nice things. And i was fully right to do so, and you would do exactly the same in my position