There are multiple reasons, but none of them guarantee that Bitcoin will remain the most valuable cryptocurrency forever.
First off, BTC has the first mover advantage. As the first cryptocurrency, it is the most recognizable and widely trusted "brand" of this new asset class, so it naturally receives the most investment.
Secondly, Bitcoin Core possesses the most prolific developers in the space. While much of the theoretical work and innovation that has gone into some top altcoins is impressive, none can claim credit for creating a system so robust as Bitcoin.
Thirdly, the network effect. Per Metcalfe's law, the network value is proportional to the square of the number of connected users of the system. As more users come online, the network continues to increase exponentially in value. Another cryptocurrency could grow its network at a faster rate, but this hasn't happened yet.