1. You need to have enthusiastic reviewers (people who care enough about a product category to review them semi-throughly.)
2. Proper reviews can take time and may need domain knowledge.
3. Competition. When there were one or two people doing reviews on some category of products, maybe the economics worked out. Once you have hundreds or thousands competing with you, the time demand may be overwhelming and not worth it.
4. If you are a trusted reviewer or site, you will get economic pressure to review a particular thing or brand you may not like very much but the money may be good. So you will begin to experience conflicts of interest.
5. If reviews are just a hobby and not a way to make money, eventually you will slow down or move on, opening a hole that gets filled up by spammers.
7. Some things are timeless (a pipewrench, let's say) and some are seasonal (consumer electronics, toys, etc). The former deserves a through review but the latter doesn't deserve as much but it may get the bulk of interest due to seasonal demand). Does it really matter if the latter's latest iteration has 2% increased battery life to discuss?
I'm sure there is a lot I didn't think of. But it's a doomed category, unless people are willing to pay for professional reviews (Consumer Reports types and other independents).