Here's why:

* Low Mass: Red dwarfs are the smallest and least massive stars. They have a much smaller core compared to larger stars.

* Slow Fusion: Their core temperatures are relatively low, resulting in much slower nuclear fusion rates. This means they burn their hydrogen fuel very slowly.

* Long Lifespan: Due to their slow burning and smaller size, red dwarfs have incredibly long lifespans. They can live for trillions of years, many times longer than the current age of the universe (around 13.8 billion years).

In contrast, massive stars have short lifespans because they burn their fuel rapidly due to their high core temperatures and pressures.

Here's a simplified comparison:

* Red Dwarf: Think of a tiny, flickering candle. It burns slowly and lasts for a very long time.

* Massive Star: Imagine a blazing bonfire. It burns brightly and intensely but has a short lifespan.