I read somewhere once that physical cash robberies is declining in popularity is primarily due to inflation rate. Most robberies aim to do it just once and get away with as much money as possible but due to inflation, the amount you need to steal to make it worthwhile corresponds to huge amount of physical weight of that cash.

Case in point, lets assume that each bill weighs 1 gram. So in 1933, John Dillinger robbed the Central National Bank for $76,000 [1] which translates to about 160 pounds worth of cash. Easy enough to carry off with a few horses.

Now, accounting for inflation, that $76,000 will be roughly $1.2million in today's value. If JohnD were to pull off that same heist to get the same value in today's world, he will need to find a way to transport 2650 pounds or 1.3 tons of cash!

Then ask yourself, is $1.2 million enough to get you by today? Surely it will give you a comfortable life but won't guarantee that you will be set for life. So, lets put that value at a "small" $5million since you have a plan to live frugally with the stolen cash. Now you will have to find a way to transport 6.5 tons (13,000+ pounds) of cash and not to mention, find a place to hide it!

So yep, inflation is the cause of the traditional bank heist being not as popular as it once was.

[1] https://en.wikipedia.org/wiki/John_Dillinger