I think it is because it is a full 35mm frame lens.
I find it quite "inefficient" to use full 35mm frame lens on my oly DSLRs. I mean you are carrying dead weight most of the time. Same for the sigma 105mm f2.8. Also full frame. Just doesn't make sense to use such an unoptimized lens design in terms of weight size and performance.
I mean if I pay the price in using a smaller noisier sensor by using 4/3rds DSLRs at least I want to reap the benefits of lighter/brighter 4/3rds lens, rather than lugging around a full frame lens
I think the better lens are the sigma DC lens.
I agree fully, except in the case of the 105mm, as for now, there is no alternative. For macro shots in the case of the 24mm, I would prefer to buy the Oly 35mm instead. I believe that lens is smaller and probably has longer working distance, which is important for macro. I find it difficult to handle the ED-50 I have because of the short working distance, so I suppose the 24mm is even worse.
Edit: I just noted it is a 1:2.7 lens. Even less magnification than the ED-50, which is 1:2. With a minimum focusing distance of 18cm, I think it is not very useful as a macro lens, especially if one wants to add the EX-25 to get more magnification. In that case the lens is probably focusing inside the lens tube. Considering the price, magnification factor, filter diameter, weight and the fact that it is a Sigma, I think I would definitevly prefer to get the ED50, or the 35mm Oly and not this lens.
