The greatest hurdle for bitcoin is the huge amount of technology and infrastructure needed to make it work, cash is just a piece of paper so it is natural that it has become widely adopted all around the world.
But even cash is not used by everyone, there are many remote communities which still use barter or that use their own currency, and bitcoin will have a lot of problems to get to those people as the requirements to use bitcoin are way higher than cash.
There is no doubt that cash is a piece of paper but it is the same piece of paper that is being used all over the world today; Some places may not use cash but still we cannot ignore the fact that using cash is common and everyone is doing it. Of course, bringing Bitcoin to remote communities is a difficult task, but with time even those people will become aware of Bitcoin. Currently, cash is more widely used than bitcoin. I think Bitcoin has no apparent existence, which makes it hard to replace cash. Bitcoin has been around for 14 years, but I think 90% of people around the world still don't know what Bitcoin is? Cash can be used by everyone and is a common everyday item, while Bitcoin is a digital currency that is not for everyone.