Well, I think it comes down to that the burbs are far more accomodating to a tradesman.
First of all, a tradesman essentially needs a work vehicle, to haul tools, supplies (such as lumber, dry wall, paint, etc...) to take out garbage (trust me there is a lot of garbage that you can not throw in the local dumpster, hehe) and owning / storing such work vehicles downtown is very expensive.
Also, tradesmen tend to work all over the metro region, downtown for 1 job, Surrey for another, Maple Ridge for another, etc...
Being downtown is being trapped on a peninsula that is slow to escape by vehicle south and east (where most of the metro and trades jobs are located). Therefore starting from a location in the burbs gives one more access to our highway system and shorter travel times to the varried work places. Again, if drving from downtown and Vancouver was a little easier and not being made more difficult every day on purpose then maybe more tradesmen would live downtown.
Remember, not everyone has a stationary job, an office job, a 9 to 5 job, or a job that does not require the transport of heavy / awkward tools and goods.
Also many tradesmen love having garages where you can work on some projects at home. and store your tools and supplies.