* They're easier to fret. The upper two strings are thinner than the lower four, which makes them easier to press down on with your fingers. This is especially important for beginners who are still developing their finger strength.
* They have a brighter sound. The upper two strings produce a brighter, more treble-y sound than the lower four strings. This makes them ideal for playing melodies and leads.
* They're better for slapping and popping. Slapping and popping are two common bass guitar techniques that involve slapping the strings with your thumb and pulling them with your fingers. The upper two strings are ideally suited for these techniques because they're easy to fret and have a bright sound.
Of course, this doesn't mean that the lower four strings of a bass guitar aren't important. They play a vital role in providing the foundation for the music. However, the upper two strings are often used more frequently because they're easier to play and have a brighter sound.