Technology. More specifically, new "public ledger" or "blockchain" technology inspired by Satoshi's implementation of bitcoin. This is what it looks like when a new important field of technology comes into being.

This "new" technology is 10 years old. Why is it catching on now?

I'm guessing most gains are from increased demand due to investor speculation.

When you look at the crypto usage for practical purposes there aren't too many yet. That tells us it's mostly people buying and holding.

The magnitude of the boom is likely a speculative bubble. The reason there is a boom of any size is that many more people are working on blockchain applications now.

I keep hearing youtube ads for apps that let you speculate on cryptocurrencies

Unbridled speculation and compound interest.

That's a pretty lazy answer. Why not last year, or the years before? Bitcoin is nearly 10 years old and monetary policy has been "easy" that whole time (in the US and Europe). Why is it taking off this year in particular? What is the deal with the slew of ICOs? I am not sure what you mean by compound interest. Are you trying to make a pun, e.g. compound hype or referring to financial interest rates?

How correlated is Bitcoin price to the stock market rise and low Federal Reserve rates?