one of the main reasons is, there are 7.4 billion people in the world, and there's several trillions of dollars on the world but there will only ever be 21 million bitcoin.
Currently there's 15 million bitcoin.
So, if we divide all bitcoin evenly over 7.4 billion people that means that every person will have 0.00283783 bitcoin on average.
That's a tiny amount of bitcoin.
Of course, if bitcoin would still only be worth $500 than most people could easily buy 1, 10 or even 100 or 1000 bitcoin.
But there's not enough bitcoin for everyone to have 1, 10, 100 or 1000 bitcoin, only a few people can have that many, and the rest will have to do with 1/1000th of a bitcoin or less.
So therefore, 1 bitcoin has to be so massively expensive that most people will not be able to afford one in their lifetime
that is of course assuming that bitcoin will be used by everyone eventually.
But even if bitcoin will only be used by 1% of the worlds population that that still means that bitcoin will reach high amount, just not as high as it would otherwise (still easily $10,000)
this.
the logic is simple. bitcoin can fail and price could drop, so you lose your invested money. which is a fixed numer. on the other hand bitcoin could succeed. mass adoption event of hyper-internet-monetization epicness is possible. that means hundreds of million or even billion of people will use bitcoin. in order to serve this demand those few million existing bitcoins
must improve in orders of magnitude. so the upside of this investment is unpredictibly high. if you ever played poker you know that this bet is almost a no-brainer. if bitcoin succeeds it will be big.