Warren Buffet said "Diversification is a protection against ignorance," I'm all on on bitcoin!
Bitcoin is the best performing asset in the past 10 years. There's no point in diversifying any further when you understand and believe in the future of bitcoin. Big gains still to come!
I think what Warren Buffet meant was that with diversification you can reduce your risk to fall for trusting too much into one idea and not evaluating the risk properly.
You are absolutely right, if you are a 100% convinced of an idea like Bitcoins and you want to go for the highest profit possible, then you should go All In and invest all your money. However, most of us here need money for our family and can't just put everything on one card. With diversification we have a much lower risk profile for our investment portfolio and if we need money in the near future, there will likely be a good asset in our portfolio that we can sell.