Here's why:
* Massive Stars: The first stars in the universe were incredibly massive, often many times larger than our Sun.
* Short Lives: These stars burned through their fuel rapidly and lived short, intense lives.
* Supernova: At the end of their lives, these stars exploded in violent supernova events.
* Stellar Remnants: The massive explosions left behind extremely dense remnants. Given the immense size of the original stars, the only possible remnant was a black hole.
It's important to note that this is still an area of active research. While the general theory holds, the exact details of the early universe and the formation of these first black holes are still being explored.
