为什么各月原油期货价不一样？正价差与负价差

[Contango & Backwardation in Crude Oil Market]
The following chart shows the price of 3 different maturity crude oil futures (WTI).
The price differential between the futures started widening after oil price crash in Feb-Mar (leading to a large contango).

For finance students who have learnt the futures pricing formula & cost of carry model, the model & theory practically does not hold in this period, possibly due to:
Carry arbitrage activity is limited due to run out of crude oil storage.
For far month futures, oil producers (hedgers) do not want to hedge at a price lower than their breakeven but rather to shut down operation.
Speculators expects possible certain price recovery in months.

This concept is also important to calendar spread futures traders.

为什么原油正价差(contango)会不断阔大？
而这如何对想要捞底的交易者的利润产生直接的影响？