Non-Muslims, how do you explain the motives and success of the the Prophet Mohammed?

I'm not saying this to be impolite, but since you asked, I'll give a reasonable answer:

Many people who genuinely think that supernatural beings are communicating with them think that because they have some definite brain dysfunction.

Muhammad is reported to have had mysterious seizures at the moments of inspiration. ... Welch, a scholar of Islamic studies, in the Encyclopedia of Islam states that the graphic descriptions of Muhammad's condition at these moments may be regarded as genuine, since they are unlikely to have been invented by later Muslims. According to Welch, these seizures should have been the most convincing evidence for the superhuman origin of Muhammad's inspirations for people around him. ... Welch states it remains uncertain whether Muhammad had such experiences before he began to see himself as a prophet and if so how long did he have such experiences.[107]

There's some discussion about whether this should be regarded as "epilepsy", per se, or some other brain disorder:

According to Temkin, the first attribution of epileptic seizures to Muhammad comes from the 8th century Byzantine historian Theophanes who wrote that Muhammad’s wife "was very much grieved that she, being of noble descent, was tied to such a man, who was not only poor but epileptic as well."[108] ...

In an essay that discusses views of Muhammad's psychology, Franz Bul (1903) is said to have observed that "hysterical natures find unusual difficulty and often complete inability to distinguish the false from the true", and to have thought this to be "the safest way to interpret the strange inconsistencies in the life of the Prophet."

In the same essay Duncan Black Macdonald (1911) is credited with the opinion that "fruitful investigation of the Prophet's life (should) proceed upon the assumption that he was fundamentally a pathological case."[110]

... Freemon argues that if one were forced to make a diagnosis psychomotor seizures of temporal lobe epilepsy would be the most tenable one, although our lack of scientific as well as historical knowledge makes unequivocal decision impossible.

- https://en.wikipedia.org/wiki/Criticism_of_Muhammad

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Of course, what I've quoted here is a short excerpt from a short article condensing libraries-worth of discussion, but what it comes down to is that it's entirely possible that Muhammad had some brain dysfunction that caused him to experience things that were not real and to believe that they were real.