To summarize they boil it down to demand vs liquidity (yeah that bit is obvious), but they characterize the differences in demand namely big name institutional investment. Again widely discussed in this forum but this chart is slick if the underlying data is valid:
As you said, this point has been excessively discussed, and I've chimed in a couple of times about how the demand and supply ratio would influence the price; A fixed supply rate means we are technically immune to supply shocks and a growth in demand, without a growth in willingness of holders to sell would have a positive effect on price. This does not mean bitcoins would be difficult to purchase or become scarce, but rather most people are seeing the potential value and putting a premium on it.
Similarly, we also see much higher net inflows to exchanges allowing crypto-to-fiat (C2F) trades compared to 2017. C2F exchanges are playing a bigger role in this surge than in 2017, when crypto-to-crypto (C2C) exchanges, used mostly by traders swapping many different types of cryptocurrency, drove more of the market. This, combined with the accumulation of Bitcoin by investor wallets that tend to hold for long periods of time, suggests that first-time Bitcoin buyers and buyers looking to unload fiat currency for Bitcoin as a hedge against worrisome macroeconomic trends are responsible for much of the current demand.
The fall of the ICO market could be the reason for this. As at late 2016 through to early 2018, new projects were springing up all around and the altcoin market was in a hype phase, most of the projects ran ICOs and accepted funds in BTC and ETH (maybe a couple of other currencies too), leading to an increase in C2C transactions. Fast forward to now, we do not have as much new projects springing up and crowdfunding is not as popular.
Bitcoin these days is more commonly juxtaposed with Gold or hard fiat currencies than altcoins, as a hedge against inflation.
Overall, the current rally is different than any we have seen (although, it would be the first I'm actually experiencing). After a couple of bull runs, investors have become more aware, and would have learnt from previous experience. They've seemed to also raise the expectations,
BTC is currently closing in on $19k with no real excitement in the market or media and also no hyper fomo yet, makes me assume we're only getting started.