Here is a guess. Some nutritients / elements are rare and plants need to manage them carefully. That also means that they need to protect themselves against an

Here is a guess. Some nutritients / elements are rare and plants need to manage them carefully. That also means that they need to protect themselves against animals feeding on them. For that, they develop antifeedants [0] for example. Maybe plants develop the structure of leaves and antifeedants first before eventually loading it with chlorophyll [1] for which e.g. magnesium is required (no idea if that counts as rare but it is certainly harder to acquire than hydrogen, oxygen or carbon. [0] https://en.wikipedia.org/wiki/Antifeedant https://en.wikipedia.org/wiki/Antifeedant [1] https://en.wikipedia.org/wiki/Chlorophyll#Chemical_structure https://en.wikipedia.org/wiki/Chlorophyll#Chemical_structure