What is so great about No Deal that it outweighs the downsides
First you need to list all the upsides and downsides, which people are unlikely to agree upon anyway, then compare. Here are my ideas:
Upsides
Government actually takes note of what voters say
Sterling likely to devalue which makes exports cheaper to others.
Imports become more expensive so internal demand for UK produced goods and services increases
UK can implement trade deals directly whereas at present they are governed by EU
Subscription to the EU will end (deal or no deal) which is worth Approx 9 Billion per year
UK might be able to reduce, or even avoid, paying the 39 Billion
Stricter controls on future immigration
Downsides
No deal likely to be chaotic in the early days as UK don't appear to have a plan for No Deal
Not paying the 39 billion will likely hinder future trade deals with the EU
UK businesses whose revenue comes mostly from EU may wish to relocate to the EU for ease of administration. Employees of such businesses may object and so become unemployed.
GFA may fall by the wayside leading to a return of hostilities
I am sure others can think of many more to add to, or delete from, both lists