That means that any security breach is catastrophic. Whereas the routine security breaches of credit and debit cards, and online banking systems, are not catastrophic.
Physical cash is of course not subject to instant theft from anywhere in the world - the theif has to be in the same place as the cash. Whereas it's basically not safe to use bitcoin on any computer connected to the internet - to do so is equivalent to leaving it in the street - rendering it hilariously unsuited to ecommerce.
Not to mention that the most convenient ways of actually getting money in and out of the bitcoin system, the exchanges, tend to have frequent security breaches and exit scams of their own.
(I should just point out that in this discussion generally the anti-bitcoin side uses 'bitcoin' to refer to 'the totality of systems needed to do normal useful things with bitcoin', while the pro-bitcoin side limits it to only the protocol and possibly the reference client implementation.)