Because it means agreeing to the divorce bill and withdrawal agreement before trade talks for a start.
TM is lying when she says 'it satisfies Brexit' because she claims it means we leave court of justice, we stop paying in, it ends freedom of movement etc etc.
It doesnt mean any of those things -that all depends on the trade talks -which will no doubt mean fishing rights, Gibraltar etc etc.
It also means the end of passporting rights -or at least the EU will be looking to snatch as much financial services as they can from the UK. Barnier tried doing that long before Brexit, but as members it got voted down.
so its a crap deal, because once the withdrawal agreement goes through we have to agree anything the EU want as far as trade talks go.
And we clearly have no useful negotiators so the UK will get the worst deal from the EU and every other country.