“Because it is faster than walking.” Nice and Cheeky!! But there is more.
If Sherpas followed one of the Abrahamic religions, Khumbila would be Mt Sinai. The mountain is held in such reverence that no one has been allowed to climb it yet. Khumbu has lots of stories of misfortune befalling those who have tried to climb it including one involving Sir Ed Hillary in his younger days.
At 5,761m, a dwarf by Himalayan standards, the mountain nevertheless dominates the landscape around major Sherpa settlements. And right at the foot of the mountain in a valley shaped like a horse are the twin sister villages of Khumjung and Khunde. Hence, when Khumbila is depicted anthropomorphically, we have him riding a white horse, because well the valley is shaped like a horse.
The custom of worshipping mountains as deities is a vestige from pre-Buddhist paganistic religion of Tibet. Upon introduction, Buddhism incorporated local customs into a larger philosophical framework rather than eliminating them. And this is especially evident in the Nyingmapa (the old ones) sect of Buddhism followed by the Sherpas. Examples include worshipping mountains like Khumbila, Pokalde and Taboche as deities and exorcistic rites such as the festival of Mani Rimdu.
Read more about Everest in Everything about the Everest Base Camp Trek