Well, ethereum is the most popular platform, so it is mainly used for building other projects on it. It is also among the first (if not the very first one) to introduce smart contracts, which allow people to perform transactions without having to trust anyone. Of course, now there are many other projects which may turn out to be faster and on which it might be easier to create smart contracts. But people already got used to ethereum and trust it, because it is really powerful and popular.