It's likely a combination of all 3 factors that you laid out in your question -- local beliefs discouraged climbing it, modern equipment made the prospect of summiting not just possible, but probable, and earlier non-Western climbs were unknown. Everest is also known as Sagarmatha in Nepali and Chomolungma in Tibetian. It is considered by the locals to be a place of the gods. In Tibetian Buddhism, they consider Chomolungma the palace of the Goddess of Inexhaustible Giving, Miyolangsangma. They think that climbers are her not-always-welcome guests, and believe that deaths on the mountain are the Goddess' way of expressing her displeasure with the trespassers. They also believe that the valleys leading to Everest belong to another holy being, a 9th century saint who declared the Khumbu region a place of refuge.
The Sherpa people, who are the area's locals, consider the mountain a place of deep spiritual energy, and one where a person must be very cognizant of their actions so as not to affect their karma negatively. When the British first decided to explore the area (after "discovering" it during the 1802 Great Trigonometric Survey of India, though it's existence and location had been mapped at least a century beforehand by the Chinese Qing Empire), they had quite a hard time finding locals to accompany them because of these beliefs. After the first British expeditions in 1921 and 1922, where they paid local Sherpas handsomely and sought (and received) the blessings of both the Dalai Lama and the head Lama of the Rongbuk Monastery (which sits in the Khumbu Valley), they found that some locals were then more willing to accompany foreigners on their expeditions. Until 1960 it was always Westerners driving and recording the attempts.

With regards to the equipment necessary, it is possible that the 1924 Expedition (where Mallory and Irvine were lost) had the right stuff, as they carried supplemental oxygen and the most advanced climbing equipment of their time. Whether they summited is unknown, but it is a possibility, meaning that an Englishman and an Australian were the first to summit (with sherpas only employed to lug gear to the lower elevation base camps). There were occasional expeditions between 1924 and 1953, but they were few and unsuccessful due to World War II, the Chinese seizure of Tibet and subsequent closing off access from the north, and the complicated process of finding a route to the base from the south through Nepal. Once expeditions began again in 1950, the climbers had many more equipment options thanks to technology advances made during the war. More portable oxygen mechanisms, better insulated clothing, more sophisticated ropes, better instruments for measuring altitude, etc. likely aided Hillary's efforts.

Finally, it's absolutely possible that people climbed Everest even before the first British expeditions, and their accomplishments were 1. unknown, 2. not recorded, or 3. recorded but not available to Westerners. Everest is located partially in Tibet, which China seized and occupied in 1950 and has been quite persnickety about access to the mountain for non-Chinese climbing teams as well as hostile to claims of native Tibetians that they view as undermining Chinese control over the region. There is archeological evidence that the Khumbu Valley has been inhabited for 11,000 years, so it's absolutely possible that ancients did climb Everest but records have not survived to present day.

Sources: Into the Silence: The Great War, Mallory, and the Conquest of Everest Wade Davis, Knopf Doubleday 2011 Life and Death on Mt. Everest: Sherpas and Himalayan Mountaineering Sherry B. Ortner, Princeton University Press 1999. Mount Everest, The Reconnaissance, 1921 Charles Howard-Bury and George Leigh Mallory, Longmans, Green & Co. Publishing, 1922.

Interestingly enough, the desire to conquer a mountain simply "because it's there," is a largely Western phenomenon that gradually spread to other countries (most notably Japan and Korea) in the 20th century. The desire to climb mountains didn't really enter Central Asian mountain communities until the influx of outsiders made portering and guiding a way to earn a living. In short, climbing just wasn't many Sherpas had the inclination to do. u/caitrona has done a good job outlining the religious disincentives to climbb as well.

Of course, there were some non-Western cultures that produced some remarkable pre-modern climbers. Incan religious sites have been found as high as 6,700m and mummified human sacrifices at up to 6,000m in the Andes. But again, these climbs were made for reasons related to Incan religion, not sport.

Furthermore, climbing an 8,000m peak like Everest is extremely difficult. So difficult, it would probably have been impossible for Himalayan Sherpas or the local people living near the tall peaks of the Karakorums.

For one, Sherpas and other Central Asian mountain peoples simply didn't have the technology. No crampons, ice axes, goggles to deal with snow blindness, climbing boots, pitons, jumars, or high-strength climbing ropes. Now, Everest can be climbed with relatively primitive equipment. Conrad Anker and his partner demonstrated this when they summitted in 2007 using replicas of the equipment used by George Mallory in 1924. However, they had an extremely difficult time of it, despite the fact they had the support of a team and were experienced climbers (Ankers was a veteran mountaineer and his partner was a world-class rock climber). But even in their case they had ice axes, suppmental oxygen, and other equipment Sherpas hadn't (or couldn't) make.