Why not every month?
The Moon's orbit is tilted by 5° to the plane of Earth's orbit (the ecliptic). So at new moon the Moon usually passes slightly above or below the Sun, and its shadow misses Earth.
Eclipses happen only when a new or full moon coincides with the Moon crossing the ecliptic — at the nodes ☊ and ☋. That happens twice a year: the «eclipse seasons».
The line of nodes slowly turns — one full turn in 18.6 years — so the seasons come about 19 days earlier each year.
View from the place of best visibility. Where exactly it's visible — see the maps on NASA's site (eclipse.gsfc.nasa.gov).
Simplified model: a constant recession rate and a slowly growing Sun. The most common scientific estimate is about 600 million years; in reality the recession rate changes.