Game theory point of view
We can view it from the game theory point of view, with a simple prisoner's dilemma situation.
Let's keep it simple with only two oil producers, Saudi Arabia and Russia.
Both can either decrease production, or sell all they can.
The less oil is sold, the more each barrel is sold for.
The best situation if we add their revenues is keeping production low, and prices high.
However, if the prices are high, one could sell all their oil. High prices, lot of production, maximum profit.
But this of course, doesn't please Saudi Arabia: the prices are high only because they produce less. They could earn more if they sold more.
We arrive to a Nash equilibrium, where both sell all they have.
Sure, the first situation would be best, but it isn't stable, as it just require one cheater to break the equilibrium.
Different production costs
In addition to the analysis from game theory, it should also be highlighted that both countries don't have the same extraction costs. It costs less to Saudi Arabia to extract oil, so they can afford it, as even with low prices, they still earn money, whereas countries extracting lower quality oil, like Russia, could lose money from selling too low.
This is however true only in short-medium range, as Saudi Arabia is much more dependent on oil selling than Russia is for their economic growth.