Some of the responses to this completely miss that "Get Brexit Done" is successful for being short and snappy.
The answer to it thus can't be an entire lecture on why FOM is good or what have you. https://twitter.com/JamesMelville/status/1203236873910542342

Like I said before, Get Brexit Done works because it appeals to both passionate Leavers and those just sick of Brexit. It targets both audiences and is intentionally vague so people can read into it whatever they want.

It also follows a pattern of similar far right phrases:
Strong and Stable
Leave Means Leave
Drain the Swamp
Lock Her Up

All crafted by people who have done specific research in effective psyops/conditioning.

Even we've used the 3 word structure
Bollocks to Brexit
Stop the coup

Something about that structure resonates with people, likely because it can be easily chanted.