Simply because it's the leftmost lane. People assume it is going to be the fastest / passing lane because of this.
I agree with the above statements however. Having a 2+1 setup in the express where trucks can use both general lanes is going to aggravate a lot of people- not just the speed demons / aggressive drivers. The express should have been 3+1 with the leftmost general lane restricted to trucks. If you're saying that's too many lanes for what's needed here, then perhaps they could have stripped a collector lane (just 2) for the extra express lane. Sure that may make for tight collector traffic but the highway may probably flow better.
...But I am not an expert in this field. However I am curious if the experts even approve of the finalized 2+1 HOV design for the express. I think this may have been simply done for political reasons as the former Liberal government wanted all new widening projects to include HOV lanes. A 3+1 HOV express was probably too expensive for them and didn't fit under existing structures (the support columns), and HOVs don't really make sense in the collectors. This was the only way to do it unless you replaced all the overpasses that were already designed in mind for collector-express. That would have added cost, complexity and construction time to the project, not to mention wasted infrastructure as these overpasses still have a lot of lifespan left.
There is "entitlement" all over the place. HOV drivers feel special, so they feel they can drive faster than the other lanes (the HTA still applies, so 130 in the HOV lane ought to be the same offense as 130 in any other lane. But in practice, OPP will ignore HOV speed so long as drivers keep their spacing and don't weave. HOV side is a riskier place for a traffic stop, anyways.)
Truckers feel entitled to jockey for an extra 3 or 4 km/h over a slower truck, which is the problem..... if they use both lanes you have the hazard of trucks changing lanes alongside or in front of overtaking autos, and once in the left lane cars may bunch up behind them, leading to impatience and further bad judgement. (eg cars then trying to pass on the right, or enter the HOV lanes in the wrong places)
Maybe they could figure out a sprayer that puts a mist of tranquilisers over the road so that drivers relax and lose the impatience and aggressiveness. ;-)