To understand why, we need to understand what a planetary conjunction, as such events are called, really is. Earth's orbit is no exception, but it's hard to tell that by looking at the sky. Historically, it was very difficult for humans to determine that Earth was moving. They assumed the sun moved and called its path the ecliptic. Because the planets orbit along the same plane as Earth, they also seem to travel on or near the ecliptic.
Conjunctions happen because the planets all orbit the sun at different rates.
Like runners moving around a track in separate lanes, the inner planets are going to make a trip around the sun faster than the outer ones. A close one like Mercury zips around the sun in 88 Earth days. Venus takes 225 days. Mars moves much more slowly at 687 days. Jupiter ambles around the sun in 11.7 Earth years, and Saturn crawls at 29.46 years.