I gues more people are interested in animals...but give the forum more time, it just started. For such a topic t is normal to take years to get a larger number of active members. People who are interested in this topic first need to find it on the web.
I agree with Peter. Peter's Forum has taken almost 1,5 years to became a popular forum. The other problem is that there are much more available information about extinct and endangered animals in books or magazines than about extinct and endangered plants, though there are much more extinct and endangered plants on our planet. And it is easier to find illustrations about extinct animals than about extinct plants. Peter's forum is part of his very popular website, so that many people came into the Extinction forum via his website. The next thing is the Wikipedia. I think if you want to write a text about a recently extinct animal, Peter's forum is one of the first addresses to find reliable information. I think it is important to build up the Vanishing Plants forum in a similar way.