I think having your van sign written is a must!
Answer to your question though: People that dont have their van signwritten are:
1). Too busy, they dont need extra work
2). Contract cleaners that MUST have a plain van
3). Splash and dashers that dont want to be caught
4). Driving around pretending their not a carpet cleaner, plumber perhaps

5). Cant think of another reason :

Its free (when paid for) 24/7 advertising and re-assurance when you arrive at a job that you're the company the customer called from whatever advert.
I get a job once or twice a month from my van (sometimes more, sometimes less), latest was week before last when at a petrol station. Lady came over and asked for a quote while i was filling up, booked her in and did the job this week.
Also jobs from neigbours seeing van working on their road.
Even had calls while driving from people behind me in traffic!
As said above, it is a no-brainer. For what it costs possibly the best one-off advertising cost you can have.
All the best,
Jason.