Why do Crimeans think so differently of Russia compared to the people of Donetsk/Luhansk?

1. The quoted source talks about people on free and recently liberated territories of Donbas who know the war and who don't want the invaders to come to their land.
2. The people of free regions can speak out freely. But we don't know the public opinion of people on Russia-controlled territories of Crimea or Donbas. Whatever happens behind the barbed wire or at gunpoint is anything but a legitimate survey.
3. Speaking of 21st century, people of Donets'k and Luhans'k have seen the horrors of war, while they who occupy Crimea haven't (yet).

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The Wikipedia article quoted in the original question says (highlight mine),

The survey contained an over sample of respondents from the Ukrainian-controlled areas of the Donbass […]

Most obviously, people on free part of Donets'k and Luhans'k regions know the horrors of war that has been raging a few dozen kilometers away and they don't want it splash to their place.

Even more so are people of liberated places like Mariupol' and Slovyans'k who have seen the invaders in their towns. Regardless of their ethnicity or spoken language, they don't want Russians to come again. See this short video of Russians shelling a peaceful city of Mariupol' to know their reasons.

On the contrary, during the first several years of attempted occupation, people on Crimea enjoyed a status of bespoke "display window", when the agonizing economy of Russia has subsidized a whopping 79% of the entire Crimean budget.

A word about the refugees. A huge number of between 1,500,000 and 2,000,000 people fled from the Russia-occupied territories after 2014. It is not known how the surveys quoted in the original question calculate these people's opinion.

Having this said, I believe your original intent is not to receive a trivial answer like this, so I'm going to re-frame the original question and post a different answer accordingly to my understanding of the question.

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Let me also correct several inaccuracies in the original question. I believe this would shape the question according to the internationally-recognized course of events.

• Crimea overwhelmingly voted to join Russia in 2014 […]

  No voting took place on Crimea after 2012 Parliamentary election. The "Occupendum" has never been recognized by vast majority of world countries or international organizations.

• Crimean public opinion is strongly in favor of joining Russia […]

  There is no credible information what Crimeans think. The last known surveys took place before the occupation, and it has not been that bad. Daresay, 50% is far from being "overwhelmingly".

  In March 2014, the Russian regular army took control of government buildings and the entire infrastructure of Crimean peninsula. Whatever happened at gunpoint is no different from voting in a concentration camp. Everyone would do what they are told to, and even then the "authorities" can draw whatever number they want, even the turnout of 146% wouldn't stop them.

  This is not the way how the polling station looks like (source: Wikipedia):

  

• After all, they are (were) all Ukrainians […]

  No. They were different factions of immigrants. This deserves a separate answer which I'm going to post soon. The indigenous peoples of both regions are Crimean Tatars and Ukrainians, correspondingly.

It is somewhat unfair to use so-called "Ukraine public opinion" from Wikipedia — because it is the fact, that creates such a gap for you.

Real opinions of people are well described by that map (support of "pro-Russian" Yanukovich, from english wiki):

And that map shows such divides in historical perspective. Note, also, that "added" — was added by the Russian Empire (before 1917), and then the Soviet Union, not by Ukraine itself. You may see similar map if opening history book, or just opening reddit

And that shows dividing by language (was picked from Wikipedia):

Correlates pretty well, doesn't it?

So, in fact, the answer is, there is NO "thinking differently", other than in Ukrainian official opinions. That country is divided in its core, with pro-Western west and pro-Russian east and south.

Since the Ukrainian Government started to suppress ethnic minorities, as Russian or Romanian/Czech in the west, the standoff began. Open and armed, as in the east, or diplomatic — with Romania/Czech Republic in the west.