One of the most simple reasons is that the market is still too new and innovative and people do not really understand it, it is my opinion that it will take at least a few decades for bitcoin to become mainstream because it takes that long for people to adapt and get used to new technologies, however lets not pretend that bitcoin is ready to be used by billions of people, it is not, as soon as the price begins to recover the fees begin to skyrocket and that is with just a couple million of users.
What do you think it will happen if a billion people were trying to use bitcoin? So before we can see mass adoption we need the problem of scalability to be solved completely.
This is where LN do sits in but for now its still on test/development phase.Scalability is one of the reasons why Bitcoin cant really be adopt completely.
Im not even hoping for it to happen though yet just to think into its entire supply wont really be enough to serve out the entire globe.
So it would be good if it would stay as it is as a digital currency.Its better to be considered as a viable option yet fiat would always still be the main ones.
It seems you are worried about the issue of the limited supply like many others, some have the idea there is not enough bitcoin to serve the whole population of the world, but this is not accurate, there is enough bitcoin but its limited supply means that it will be extremely valuable, if bitcoin really becomes adopted worldwide by billions of people I will not be surprised if just a few bitcoins were enough to consider a person to be extremely wealthy.
On average if every person of the world adopted bitcoin, a number that is currently at 7 billion, and we distributed the 18 million coins currently in circulation each person will only get 0.002571BTC, while the numbers are unrealistic this should give you an idea of how valuable bitcoin could become in the future since a person holding one bitcoin will have almost 400 times more money than the average person.