The best reasons are:
No physical currency involve!
1. No need hefty fee to make a transaction
2. easy to access them
3. etc
It's most common joint statement can make a bitcoin is the best future currency, so what happen with bitcoin for some years I can't predict it but just a simple statement to make a different situation if Bitcoin has a rival. Can make bitcoin is not a best future currency. For now, many people need a more sophisticated and straightforward money, and then Bitcoin has it all.