Why would it be particularly easier to manipulate bitcoin pricing over other stocks/securities? Simply because the authorities don't understand it yet, or something to do with the nature of crypto-currency itself?

Because there are relatively few buyers and sellers in the market and the amounts of transactions are small, so one big mover can shift the price a lot.

According to some blockchain site[0] the average transaction volume for BTC is something like $2.25m USD equivalent

For comparison, on an average day, SPY[1], which is just one security (albeit a popular one) trades something around $17,160m (e.g. $17.16b).

[0] https://blockchain.info/charts/estimated-transaction-volume-...

[1] https://finance.yahoo.com/quote/SPY

Because there's fundamental value behind most traditional securities, even if it's well below the current market price. If you hold a bond, it will (usually) pay interest and principal. Equities carry the value of the cash holding of the company, plus the potential for future dividends. These cash flows act as an anchor on the value of those securities (in both directions, up and down.)

Couldn't you say the same about penny stocks? And won't this problem go away as BTC gets more popular?

> Couldn't you say the same about penny stocks?

Sure, but there are also laws and regulations that limit what you can do with stocks on a real exchange: https://en.wikipedia.org/wiki/Microcap_stock_fraud

> won't this problem go away as BTC gets more popular?

Maybe. It would have to get a lot more popular, and a lot more legitimate and regulated. At that point, it's lost all the cyberpunk appeal, and it's just another speculative asset.

Regarding penny stocks, this does happen all the time, although its expressly illegal (look up boiler room scams).