Because the UK chose to introduce red lines that are contrary to their obligations under the GFA
Red lines were necessary to respect the vote to leave. However, I would agree that not all of the red lines were necessary.
Leaving the current customs union and single market were necessary to allow UK to make trade deals with non EU countries without hinderance from EU. Remember that 54% of UK exports are to non EU countries and a trade surplus of £44 billion with non EU countries. Likewise UK has an ever increasing trade deficit with the EU since 1999 and current stands at £64 Billion. Not a good deal for the UK considering it is the third largest donor to the EU and purchases from the EU make up 8% (1/12 of EU trade even though there are 28 EU members)
Germany is the largest donor to the EU, but has a massive trade surplus with other EU members that easily outweigh their contributions. So Germany is delighted with the EU.
France is the second largest donor to the EU. They break even with EU.
If UK was in the same position as; France, Germany or one of the 18 EU members that are net receivers of money from the EU, I too would be a remain supporter.
The red lines about freedom of movement is one I would drop. UK, and any other EU member, can introduce legislation that requires persons to meet criteria before being allowed entry into UK. Even before the referendum UK was tightening regulations for Immigration. People either had to have a job or means of supporting themselves.
The disconnect between GFA and Article 50 is the issue to be resolved. In pure legal terms one, or both, would be modified to ensure alignment. However, that ship has long sailed as GFA was passed in 1998 and Article 50 was passed in 2009 and subsequently been invoked.
So one has to bend. As Article 50 was produced 11 years after the GFA it would be logical and fair to bend Article 50 (that is my view based on Arbitration experience).
GFA has been proven to have achieved its goal of peace on the island of Ireland. Whereas Brexit is an unknown that has no history and the outcome has yet to be seen. Hence further argument for bending Article 50 (again that is my view based on Arbitration experience).
but they can refuse to collude with them in a deal that would break the GFA
If UK MPs buy into EU suggestion that NI remains in the EU customs union there are no issues with GFA. I support the EU suggestion, but will it get past;
DUP
ERG
SNP
Labour
And the sky is blue
Does that include the period between sunset and sunrise? When walking back from Healy Macs (a popular Irish bar) to my apartment at 11pm last night the sky was black with a few white specks.