Read the rest of Article 50, especially paragraphs 2 and 4. The last thing the EU respects is a member state’s “own constitutional requirements”, and the reaction of the EU at large to the referendum speaks volumes. Invoking rebus sic stantibus does not give any advantage to the EU but subjects it to the laws of treaties, which they have violated multiple times; it also means that nothing within the treaties can be binding on the UK once invoked, whereas Article 50 binds the UK.
I think you may be giving the principle of
clausula rebus sic stantibus more status. Britain cannot simply invoke it and miraculously be out of the European Union - the EU could continue to insist Britain is a member unless and until the case for the claim went through the Court of Justice of the EU. While Britain might choose to ignore those claims, other nations would not.
Maybe the United States under President Trump would be willing to negotiate a separate trade agreement with the UK under those circumstances, but most of the non-European countries the UK relies on for trade would likely not do so, and the EU nations certainly would be highly unlikely to. And that process could drag on for years.
While Article 50 does contain provisions that don't make withdrawal entirely straightforward, paragraph 3 means that once it is invoked, Britain is out in two years unless it agrees to an extension. That gives far greater certainty than potentially years of litigation in CJEU.
Like I say, I'm not an expert on this - I know British constitutional law very well, but the intricacies of the EU are something I haven't studied in detail - but people I know who are experts on both and who I know genuinely want Britain out of the EU, are telling me that Article 50 is the best approach. They could all be wrong but one of them has actually said that if clausula rebus sic stantibus clearly applied, article 50 would never have been necessary in the first place. The EU was designed to try and lock nations in.