I think that this is mainly due to the fact that banking as we understand it didn't really exist before this point. The closest thing at the time was giving your gold to a goldsmith who would store it in a private vault and give a receipt in return.

Even when banks did start issuing the promissory notes which would eventually become paper money, they were issued on a bank by bank basis. The value of the notes in your pocket would decrease the further away you got from the issuing bank, because whoever you gave it to would have to ride a horse back there to collect the gold it represented.

In addition to this, there wasn't the availability of things like like high explosive, getaway vehicles or even decent steel tools that the modern miscreant enjoys. This would make breaking into the vault a long and noisy process with no easy way to escape.

The upshot of all this is that a thief could either get promissory notes which the banker probably remembered being stolen and were therefore worthless, or they could make a risky attempt to steal very heavy gold in a very heavy lockbox which would take a long time to break into.

It's only with the introduction of universally accepted and standardized banknotes that bank robbery becomes a worthwhile crime. Otherwise it's just like stealing vouchers from a store and then having to return to spend them.