Why are Venus transits so unpredictable?
At first, you might think the "schedule" of Venus transits sounds pretty random. But there is a pattern:
for a total cycle of 243 years.
So why don't we see a Venus transit more often? Several reasons.
Since Venus orbits closer to the sun than Earth does, it has a shorter distance to go to make one "lap." For every 8 laps Earth makes, Venus makes about—but not exactly—13 laps. That means Venus passes us up five times about every 8 years, or once about every 1.6 years.
But we don't see Venus lined up in front of the sun every 1.6 years because Venus's orbit is tilted a bit from Earth's orbit. So when we would expect to see Venus passing us up, it is either above or below the Sun, so we don't see it. We see it only if at just that time in its orbit, it happens to be crossing Earth's orbit.
Also, neither orbit is exactly circular, which throws off the rhythm of the dance just a bit more.