I head up customer experience at Re:amaze and we work with merchants on Shopify, Bigcommerce, and WooCommerce. These alternatives all have something to offer that is unique to a particular merchant's business preference. Shopify is much easier to set up and offers more direct access to marketplaces. Bigcommerce is a bit more powerful for high volume, high GMV businesses looking for headless commerce. WooCommerce is, of course, low cost and works with WordPress and that's a huge plus for many merchants. These are very basic differences but merchants should look at every platform to see which one suits them best.

Well for WooCommerce, the big selling point is likely that it can be self hosted; you don't have to use their hosting platform if you don't want to (and as far as I know, they don't directly offer one anyway, WordPress.com/Automattic does).

That's its own advantage for some people. It means you can sell in countries the US has trade restrictions against, integrate your shop with your WordPress site and modify the source code to your heart's content. Can't really do that with Shopify.

As for the hosted platforms? Eh, I guess like with anything, the market is still big enough for multiple players, even if one of them has a huge percentage of it overall. Google Cloud Platform, Microsoft Azure and Rackspace still manage to do well, even if AWS has more users. GitLab still exists despite GitHub having far more of the market. Etc.

BigCommerce, from what I've heard, offers a lot more out of box, where as you have to accomplish the same thing with apps in Shopify.

WooCommerce, Magento, Spree, etc. are often chosen for customizability, since Shopify doesn't always serve every need (even with apps). And for control over the platform.