In my point of view it is because of exothermic reactions involved.
Endothermic reactions require heat energy while exothermic reactions release heat energy.
Because with the time of operation, the possibility of coke formation and fouling in a horizontal bed reactor is less evident than in vertical one, because the heat release in a horizontal bed has less distance to travel, a temperature peak be will less pernicious than in a vertical reactor due to the exothermic reaction involved.
In other words, in a vertical reactor the temperature peak has more distance to travel.
This consideration is based on my experience, there is nothing of scientific in this thesis.