As it seems the difficulty lags behind the price increase or drop.
Could you please tell me what is the main reason for this phenomenon?
Phill gave an excellent explanation of what is happening right now, but the long-term causes are plenty, the most important and most ignored fact is that price drop does not lead to negative mining returns.
People have this narrative that all miners are mining at close to break-even which isn't the case, when price drops, all miners earn less, but the majority don't go into a loss zone right away, and of course, miners would settle for less profit.
Those who go into the negative territory will stop mining but their gears won't disappear, they will change hands and that's all about it, this is one of the reasons why the difficulty chart isn't moving in a vertical line and it does pause, correct and continue up.
One would say, but will some gears die and thus won't be reallocated, while that's true, many gears don't die for good and they are fixed, so just like the above, they disappear for a while and come back online in a different shape or/and a different location, also since most gears that die for good are usually pretty old, and thus their hashrate is low which makes their disappearance hard to notice if 10,000 *S7s die today, who cares?
The second factor would be the efficiency improvement, if the technology reached its peak and nobody can make more efficient / cheaper gears than what we have now, eventually we will get to the point where price drop will really affect difficulty, but until then - it's not going to happen.
What people should understand is that many, many people have free or super cheap aka almost free power (my self included), I will never ever turn off a mining gear because it isn't profitable, why? because it can not be "NOT" profitable, it either dies on me or I would sell it, the latter, of course, will fall under the reallocation group which only affects the difficulty for a few days or weeks.
And then you have those people who mine for a loss, yes there are a bunch of people who mine for a loss, some do it thinking price will go up in the future ( you can argue that they should directly buy bitcoin rather than doing that but they have all different reasons not to agree to that logic), there are also people who mine to launder their money or to own bitcoin without having to use their credit card because it's illegal/expensive, and of course, some people want "fresh" bitcoins.
I can write another hundred lines of reasons why difficulty goes up while the price goes down, but I think I have mentioned the ones I think are most important.
Now that is the reason why the price goes down while difficulty does not, the other scenario of why the price goes up and difficulty doesn't keep up, it's simply because money moves faster than mining gears by a few orders of magnitude, a rich guy like Elon can tweet something positive about bitcoin or drops a couple of billions into bitcoin and price goes up 10% in a day, 10% of the total hashrate right now is 16EH, that's the equivalent of 145,454 S19 pros, that number of gears is impossible to manufacture in a day, even a week or perhaps a month, even if Bitmain had an unlimited supply of money they will be bottle-necked by something else.