There are many reasons for this dominance. In its early days, the relative ease of use compared to technical power attracted bloggers to the platform, and its open source nature attracted a broad developer following. That led to web hosts providing one-click install capabilities, and designers creating a vast number of free and low-cost themes.
Certainly another reason though is the universe of WordPress plugins—helpful add-ons for site management, customization, content development and promotion, email list building, connections to other applications and online services, and much more. Even better, most are free, or at least have free versions. And the fee-based plugins are usually worth paying a little extra for.