All of the following are reasons for greater biodiversity in the tropics, except

Frequent glaciations occurred in temperate zones (not in tropics). Speciation is generally a function of time, unlike temperate regions subjected to frequent glaciations in the past, tropical latitudes have remained relatively undisturbed for millions of years and thus, had a long evolutionary time for species diversification, 

Tropical environments are less seasonal, relatively more constant nd predictable. Such constant environments promote niche specialisation and lead to a greater species diversity.

There is more solar energy available in the tropics, which contributes to higher productivity; this in turn might contribute indirectly to greater diversity.