[jimmysong]

Because Bitcoin is credibly scarce. In 10 years, Bitcoin will have 20+ years of history behind it. Your BTC2 will be much younger, probably have had a creator, a foundation that directs where it goes, possibly changed the proof-of-work and supply schedule a few times and possibly picked winners and loser on their ecosystem. In other words, your other currency won't be that different than normal fiat money.

Most people don't really get that Bitcoin is unique in this regard. Literally every other coin has less credibility and more intervention and a shorter history. That's why BTC will be more valuable.