The first thing that always comes up as an answer is cheap electricity, but is it that cheap and a good enough reason to do business in a country with an authoritarian regime and hostile attitude towards Bitcoin? The electricity rate in China is certainly lower than in most of the EU, but there seem to be tons of countries with even lower electricity rates, some of which seem like viable options for mining farms (Ukraine, Georgia, Turkey etc.).
The first mistake is to look at that table that gets really really annoying and talk about national electric price averages!
There is no such thing as the electricity price in the US, nobody (well maybe some random 1%) pays the average, once you break down states is like this:
https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_6_aThen you go one step further and look at prices per county. Then one more step and grab a nice deal within that county, better than the average there!
Do you think miners in China were mining at 10c/kWh? No way in hell!
Second thing is that as NotFuzzyWarm said, they are Chinese, you obviously want to run your business in your own country not seek residency and permit, and all the other stuff in some foreign country with all their laws.
And third, regulations! It's one thing to run a mining farm in China, where you can do your own wiring and not care about inspections, and a different thing to try this shit in the EU. By the time you finish with the paperwork you two more halving have occurred and Bitmain is already at S61 with the gear. And all this paperwork costs money, money, and time which are essential.
Mining farms have not moved away previously because there was no incentive to move, if someone would have offered them no regulations, zero tax, 1cent/kwh unlimited power, and free citizenship they would have moved a long time ago, but that was not the case! Bitmain tried to expand its business in the US with multiple projects, it wasn't that easy as they probably imagined!