I have collected some reasons through these years studying bitcoin:

-Is sufficiently scarce to be valuable

-Is divisible enough to support a global economy (21B M1 in 2009)

-It a functional number to work inside the protocol.

-Mathematically 21M fits the needs of halving every 4 years and time enough for a global adoption and distribution of every BTC around the world.

-Im sure there are more reasons to come and discover!!