Many reasons why bitcoin is the most popular cryptocurrency first because bitcoin is the first created cryptocurrency. Because of reliability, secure, safe and have potential in many aspect. Also because of good performance of bitcoin it can now use as a payment in restaurant in some country. Also, bitcoin get the most trusted and support by the users and investors to become a part of this bitcoin society, that's why it is popular.
Yeah it is the first crypto currency ever, in past people suffered a lot for not having any kind of digital currency and they were not able to send money safely all around the world, but now just because of bitcoin people now have something really amazing and interesting. Now people are able to use bitcoin to transfer all around the world and they can, make bright futures from bitcoin for holding bitcoin as a assets for long time. So bitcoin is now popular I hope the inventor of bitcoin Satoshi must feel proud to see his invention being so popular and spreading all over the world.