When did Buddhism become in the majority Atheist?

I know it wasn't always that way. Reading books on Buddhism from as little as a hundred years ago one can see that even Zen Buddhism in Japan believed in a myriad of deities and saints. Buddha spoke of the Deva's and Brahman in the Dhammapada, and most schools of Buddhism have venerated Hindu Deva's and other deities. Examples: Ganesha, Shiva, Parvarti, Tara, Hotei, Kuan Yin, Krishna, King Yama, Jizo, etc.

Buddhism is not inherently related to any religious belief. You're comparing apples and oranges.

When Buddhists were looking for answers and is finding none (currently).

I thought that Hinduism and Buddhism were always quite polytheistic?

Buddhist are not atheist if being an atheist means not believing in god(s) or anything supernatural. Buddhist believe that there are higher beings (Deva's) that are an equivalent to god(s) of other religions. However, these god(s) are not the creator nor do they provide the final liberation from suffering.

These gods can interact/interfere for good or for bad with human to a certain extent, as long as the karma of that individual human permits it. Those deities that you have mentioned are respected by Buddhists by virtue of their karma. Even Jesus, Yahweh/Allah (in fact, they are equivalent to Brahma of the Hindus), can be respected, but they are not the final solution to our sufferings.

They aren't atheist but they are interesting.