TikZ (together with its PGF backend) is the most widely used picture drawing tool by regulars here, having more than 50x as many questions as for Metapost, alongside a wealth of documentation and user support elsewhere. TikZ is supported by several of the most popular TeX engines, while Metapost has native support only in LuaTeX, needing invocation as a separate program from other platforms. And TikZ uses familiar TeX syntax, not requiring another macro-based language to be learned.
Yet Metapost has a band of loyal followers. There are "traditional" reasons for this: Metapost is more "Knuthian", being derived from Knuth's Metafont and by report is Knuth's preferred tool; Metapost is rather older than TikZ, and so some prefer it for reasons of sticking with the better-understood tool.
Are there other reasons why users might prefer Metapost to TikZ?