USD paper bills weigh 1 gram. For about 22lbs, you can have:
$200,000 in 20’s
$500,000 in 50’s
$1,000,000 in 100’s
For all intents and purposes, you can really only rely on 20’s or less, since the $1’s would weigh way too much and $20’s are the second most widely circulated. In addition, most bills above $20 are more scrutinized and therefore riskier even if more convenient. Moving any sizable amount of physical cash is problematic on a number of fronts.
Elephant tusk goes for around $1,500/kilo. A typical tusk weighs between 23–45 kilos, so taking the mid-point that’s 34kilos, that’s $51,000 or 5 lbs. of USD using $20 bills and 27cm or 11 inches tall. Each one of these can be traced by various means to some extent, but most all it is a big problem to move around that bulk. Not to mention, relatively easy to steal.
BTC is one transaction on your cell phone. Hard to trace and hard to steal.
Illegal activity in BTC in 2018 was about $72billion or 46% of all transactions (https://papers.ssrn.com/sol3/papers.cfm?abstract_id=3102645). Money laundering alone accounted for only 2%-5% of global GDP or between $1 and $2 trillion across all national currencies. Illegal activity would be hard to assess given the variety of laws, but even if we said it was 5 times what money laundering alone was, the comparative scale of purpose makes just one crypto-currency an unrivaled champion in the space and other reports indicate its growth in this sector is accelerating and expanding (as NYT article points out).
So, think about the limited reach/accessibility and the short history of BTC and You Know What? Bitcoin’s first success story was Silkroad and its successors indicate its expanded use is brightest on the wrong side of the law.
BTC accounts are not associated with any PII, so I’m not sure what good having a BTC account does for any “authority”. Apart from anonymity and traceability, any reasonably capable user can generate and layer as many accounts as they want and quickly liquidate and make the transactions effectively untraceable.
Hope this helps your understanding of the subject.