Because it was a truly stupid thing to say. She's basically putting forward the idea of leaving with no deal as a serious alternative option that we're considering. The problem with that, is that leaving with no deal would be outrageously destructive. First of all it would leave around 1.5m Brits currently in the EU, and 3.5m Europeans living in the UK completely fecked. We'd have no legal justification for our residence, employment etc etc. This isn't some fantasy, once we leave the EU anyone who doesn't already have permanant residency (and that takes 5 years generally) would basically have to go home unless a deal is made. Funnily enough the idea of millions of people being driven out of their jobs and homes is not particularly attractive, especially for those of us who would be directly effected.
Secondly there's the small matter of trade. Without a deal, suddenly there are not only tariffs on everything, but also customs inspections. We don't have anything like the infrastucture to deal with customs inspections on the volume of trade we do with Europe, not even close. So you'd be looking at months of endless queue's in and out of the country, with businesses across the UK crawling to a standstill because the endless list of parts and supplies they buy from the continent aren't arriving on time.
Oh and its not just one way, all the things we sell to Europe are subject to EU regulation. Which suddenly we're not going to be a part of anymore. So instead of our goods and services just being waved through because we're part of the same regulatory frameworks, suddenly we're going to have to prove that our goods and services comply with EU standards.
It's an almost unimaginable horror show scenario, that would drive the UK to its knees. The idea that the ridiculous robot in Downing Street and her merry band of clowns and feckwits are actually considering it as an option is beyond insane.
Technically though she's right. A bad deal (for instance insisting every British citizen go and kill themselves) would indeed be better than no deal. The idea that anything she would potentially negotiate will be worse though, is farcical.