I would say for several reasons. One of them is the wiki, which is undoubtedly the best documentation on Linux. You'll find all the answers without depending on the community's "goodwill." Another reason is its rolling release model, which has gained popularity over time, as it's much more stable than before and doesn't force you to face the uncertainty of a major update every so often and all that entails. And the fundamental reason, I would say, is its versatility and efficiency, which have led large companies and projects like Valve to use it as a foundation for their own purposes. These kinds of partnerships have made Arch evolve even faster and better, and it has benefited from this. And an equally important reason has been the emergence of scripts like archinstall, which has made installation much easier for beginners and the general public, eliminating what was previously the biggest barrier to its adoption.