Olympus is a smaller company compared to its competitors and pretty much everything stems from there. Even if they would be represented equally, there would be fewer Olympus reviews in absolute terms because they have fewer models.
More importantly, Olympus has a smaller budget and has less to spend on review units. Each company has a pool of units which they send from site to site, including newspapers, for TV segments and select photographers. These units get reviewed by going from place to place. Depending on the company, it can take several weeks to months to get a review unit.
There is also location factor in that companies do not have offices in all countries which make it difficult for some review sites to get certain brands because logistics are more complicated. Olympus is one of those that makes the effort to have units sent across borders which puts them ahead of other small players in terms of reviews written.