The majority did after all vote to leave, if we can revoke this decision then what's next?
Decisions are 'revoked' all the time in parliament when laws are repealed or amended for ideological or practical reasons. While I can see a certain logic in that reason it doesn't seem, to me, to have much to do with reality.
I think that another reason that these Remainers are gallantly defending 'democracy' is that they don't realise how corrupted the referendum was. Most people aren't in the social media bubbles which publicise it and it has got practically no coverage in the mainstream media.
6 million for a petition is an extraordinary achievement and represents a massive depth of feeling. The various campaigning organisations, such as Avaaz, Change.Org etc. think they are doing really well if they get into the 100 thousands with their petitions.
It is in no way comparable with voting...