Isn't every deal by definition going to be worse than Remain, since all of them will result in reduced access by Britain to the resources and market of the EU? No Deal being losing the most access of all of them. That's what leaving kinda means, that we don't have to follow the rules of the EU market except when trading (when you will always have to follow their rules and they ours) with them but in return we don't have access to the resources of the EU infrastructure.
The 'deal' by Theresa may wasn't even an actual deal deal, it was just a reaaaallly long agreement to keep as close to status quo as possible until they could work out an actual 'free trade' deal, which would still be worse than Remain since still mean our products follow different standards by default and require extra verification before being traded. It's just mean we wouldn't have the WTO-default 3rd country export/import taxes on top of the cost of that certification of all goods we export/import.
The sticking point being if it couldn't be worked out what the heck do you do with the Irish border-that-can't-actually-be-a-border. If memory serves, the weird half-in half-out NI slightly different regulatory to rest of UK backstop solution they settled on was actually proposed by the UK negotiators in the first place, hence the EU being a little pissed off that the thing they view it that they compromised on more than they'd like is what parliament opposed to.
And if they open it up again, it opens everything up again. Which means France and Spain, who think they compromised too much and should have pushed more on fishing rights and Gibraltar, get to start pushing again.