Why can't the EU just reopen the withdrawal agreement so a new deal could be agreed. It can be put to MPs where it may pass.
Because of the signals which the Johnson government has been sending over to Brussels; this agreement is the result of the UK dealing with 27 nations, and getting the approval of 27 nations takes quite some time. It took over two years to negotiate this WA, and all after it happened was no, no, no - by the parliament. This agreement was the result of several red lines on both sides - of course, the EU also wants to protect itself and it won't allow somebody having the major benefits of the EU without conforming to the major liabilities as well. If the EU would grant such a thing, it would make itself weak and obsolete. And to not forget is was the UK which triggered A50, so it's up to the UK to display some flexibility right now. Aside that, why go back to the table, when the government again won't be able to get it pass the parliament anyway?
The signal the Johnson administration send to Brussels is that it is now a hardcore Brexit administration, and that they don't really care about compromises. A new agreement though would require the ability to listen to each other and to make compromises - both are abilities, which Theresa May had, but Boris Johnson did not display so far.
So far, the image Johnson sends is vastly different - he's demanding, demanding, demanding - and doesn't care if his demands are possible, or not.
Johnson also crushed the tradition of any new head of government, which is to first visit the most important neighboring states to say "Hi", so for example France. Instead he went to Scotland, Northern Ireland and Wales. This is a really strong gesture, namely "We don't need the EU" and "We don't care", which of course decreased the willingness on renegotiating as well.
For the EU to renegotiate the signals coming out of London must be different - Johnson is doing all in his might to don't go back on the table too soon, because his belief is "delays don't make deal - deadlines do."
Aside that, the EU right now is in between two terms as well, so also now occupied with itself quite much.
And last, but not least, of course the EU is setting a warning example here to showcase what you are going to loose, if you are leaving totally. Both sides are going to loose, but it is the UK which started the mess and it's up for the UK to sort it out.
The backstop is about border control; of course, the EU wants to protect its outer borders as the UK wants to achieve. I mean, one reason why many voted for leave was to get rid of the freedom of movement, taking back control on your own borders, so you cannot blame the EU wanting the same thing on her own borders.
In idea, the backstop is just a temporariy solution, until a real agreement has been made. So this means that the MPs don't have much trust in the government's ability to ever come to such a thing. And all the super duper technical solutions some people are fantasizing about is simply not yet there.