I'll take a stab at this, although it really does deserve an economics stack exchange
Short reason: Population growth relative to gold supply.
A gold standard is another way of saying that your money supply is inelastic. Each bank note is linked to a fixed amount of gold and in theory should allow you buy that amount of gold. As gold is produced by supernova nucleosynthesis, the gold is "created" only in so far as it is mined from crust deposits and the rate of newly mined gold isn't nearly enough for the level of global economic activity.
New gold is about 4% of what's needed each year (if ~135 billion USD of new gold/year and 3.9% of ~85 trillion USD of gross world product *); and most of this gold doesn't circulate as it's required to absorb commodity shocks such as oil and secure the currencies of many central banks.
As the world population grows, the money needed to reflect transactional IOUs between people increases. Eventually the gold backing the bank notes is so small and abstract as to be effectively irrelevant (no gold in your pocket), except as a concept of redeemability that still lingers in the popular imagination today.
The break away from the gold standard (i.e. that you could in theory buy a fixed amount of gold invariant of market demand for each bank note) occurred because of the need to rapidly expand war-time debt. Which is to say, compressed and time-shifted future tax revenue ("debt") resulting in a massive spike in money supply to reflect the real world massive spike in material effort involved in war activities (i.e. people do more in a total war, at least as so far trapped liquidity is concerned).
After each world war, the money supply couldn't be contracted to a fixed ratio of gold (each note's supposed "real" value in gold) without causing deadly deflation**. The Bretton Woods agreement was not actually a gold standard but rather a lip-service to one, as only central banks had access to a fixed gold exchange (i.e. not the higher volatile market rate).
Expensive proxy wars and a rising global standard of living eventually grew the money supply beyond the ability of even central banks to meaningfully exchange gold-backed promissory notes. Hence the final vestiges were broken in the "Nixon Shock" to order to allow free floating fiat currency. The shock itself is somewhat overstated considering that alternative would have required a complete re-evaluation of the role of money and a move towards a post-growth global economy which we were not and are probably still not ready for.
The core long term driver of economic growth (money supply) is population growth; as productivity growth is trapped in an upward siphon and hence barely drives the economy relative to population growth***.
Even longer reason:
* Despite the importance of money supply statistics, no one actually has anything more than a rough estimate of global M0 or MZM. Which should scare the shit out you.
* * Inflation makes a gold standard seem sexy, but deflation is far worse in our current economic paradigm of resource distribution; and at least hyperinflation is politically self limiting.
* * * Speculation and debt can also expand the money supply, but they eventually have to "latch onto" someone's actual production of work if they are to have any meaning in the first place. Hence their expansion of the money supply is closely related to the available global labour pool as influenced by population growth.