According to the film's official novelisation, the Mogwai were the result of a genetic experiment that went badly wrong. Only one in a thousand Mogwai retained their sweet and innocent disposition.
Soon after these first departures it was discovered that Mogturmen’s
creatures were highly unstable. To be exact, fewer than one in a
thousand retained the sweet disposition and charitable aims built into
it by the inventor. Instead, something went wrong. Very wrong. The
Mogwai himself knew of the unstable Mogwai, being well versed in the
historical background of his species. He preferred not to think of the
complications that had developed, but it was nearly impossible not to.
It was, after all, part of his heritage. Closing his eyes as he
relaxed in his cage awaiting his supper, he mused briefly on the wars,
landslides, and famines that had taken place on Kelm-6, Clinpf-A, and
even here on Earth because of his creator’s miscalculations and
willingness to disseminate an untested creature. Small wonder
Mogturmen had been punished by having his . . .
The Mogwai pushed the thought from his mind. True, Mogturmen had
failed in the overall, but he himself was one of the successes, the
one in a thousand who still embodied all the good things put there by
his high-minded inventor. Yet his existence, he knew, had no long-term
benefits for society. Gentle as he was, he was a distinct threat to
those around him. Just a few drops of water, a morsel of food at the
wrong time, and—
You may wish to note that Gizmo isn't the only "minority Mogwai" on Earth. There are apparently three others.
He had seen it happen before, as had the three other minority Mogwai
on this planet, and the results were nearly always disastrous. The
most recent episode, not caused by a spawn of Gizmo, had occurred late
in 1983, when a single Mogwai somehow got aboard an American space
shuttle craft, Columbia. Because of strict government secrecy, details
were never published concerning exactly how the Mogwai was allowed to
reproduce, feed after midnight, and turn into a Gremlin. In any event,
the Gremlin eluded capture by the six-man crew long enough to shut
down the computer handling the craft’s guidance and navigation
systems. When the scientists switched to the number two computer, the
Gremlin found a way to cause an overload. It then got into the system
that senses the ship’s acceleration, position, and angle of attack.
Over the Indian Ocean, Columbia actually started falling out of orbit
and was out of contact with Mission Control for forty-five minutes.
During that hectic time the pilots and scientists managed to pursue
the Gremlin into a storage compartment and kill it. Returning to earth
eight hours late as a result of the Gremlin’s meddling, the crewmen
were debriefed by government officials, who warned them not to
describe what had actually happened on the mission.