To my friends @IanDunt and @Usherwood - I would argue the explanation is in the law concerning #Brexit transition (short thread)
There are at least three legal bases on which you can construct a transition, each with its pros and cons.
1) Prolonging Art. 50 - the pro here is legal certainty: you are a full EU member, which is also great for third party FTAs. The con: full EU membership, EP elections, you need consent
2) post-dating the withdrawal agreement as proposed by @Ortino_Federico and me. Can be done with QMV, UK would remain a member, but offers certainty of membership ending.
3) an agreement based on Art. 50. This is the go-to solution for everyone, as it resolves the problem of EP elections that are coming up, the UK would also no longer be an EU member on the day of its coming into force.
Now this comes with a problem attached: legally you cannot repeat this solution. The legal basis for this agreement is Art. 50. Art. 50 grants the EU the competence to do a withdrawal agreement. But once the UK is out, you cannot do another withdrawal agreement.
To prolong this agreement, you now need to look for a different legal basis. And there's a problem - a status quo agreement would be mixed. So a longer transition will either have to be agreed ab initio or you need to choose prolonging Art. 50.
