The standard McD's defense needed to meet was "establish[...] whether commercial exploitation of the mark is real".
"However, the purpose of the provision requiring that the earlier mark must have been genuinely used ‘is not to assess commercial success or to review the economic strategy of an undertaking, nor is it intended to restrict trade-mark protection to the case where large-scale commercial use has been made of the marks’"
So the use of the trademark needs to have had some meaningful effect for the commercial activities of the mark's holder, even if unsuccessful or in a niche market.
They sent as materials:
1. A signed statement by UK/FR/DE managers how this mark is important and in use.
2. menu templates with no prices, and no information where and when these are used, and how their circulation looked like
3. packaging material with no information where and when these were used
4. marketing materials, incl copies of their websites, again with no prices, time frames, distribution data
5. sales numbers: "more than 75 Million per year" (not mentioned: where, and why)
6. a wikipedia article (might have been edited by McD or their counsel and reverted immediately by another editor)
Based on that data, a reasonable scenario could be: 10 corporations across the UK/FR/DE region ordered a total of 350000 sandwiches per work day from McD for their workforce, served from their cafeterias. That's the >75M items sold. Since they didn't care what kind of sandwich, McD sent Big Macs. Impact of the trademark: 0. The packaging and marketing material might have been internal prototypes that were never used. Impact of the trademark: 0.
From reading the decision, McD never bothered to make the case that any of that material they brought up as defense mattered in any way. The strongest one on its own was probably the "yes, Big Mac is important to us, pinky swear!" testimony by the managers, but the inherent conflict of interest reduced its impact quite a lot. The other ones? Yeah, sheets of paper or cardboard with "Big Mac" on it - so?
If McD doesn't care enough to mount a useful defense, why should the EUIPO care? It doesn't have to protect trademark holders from their own active malcompliance.
The USPTO handles trademark matters in a similar fashion, by the way: USPTO TBMP 803 states "Rather, one judge is assigned to read the testimony and examine the other evidence of record, discuss the case with the other judges, and then draft a decision and supporting opinion.", so they also decide only by the evidence that is on the record, with no requirement to take into account "common knowledge" or anything like that to help any side of the dispute (IME courts do give hints that you could ask for $keyword at $office or that $paperwork might help provide a clearer view onto the matter, but I don't know how EUIPO handles that).
Of course, there's an appeals process, and McD will take it, and they'll exercise more care in making their case this time around, and the mark will likely be reinstated for sandwiches and potentially also for animal based food.
The EUIPO treats McD the same as they treated Centrotherm, even though they're very different companies in every way. Just like they're supposed to. They might consider such consistency more important than the question if a sandwich's name has legal protection or only de-facto protection (by everybody actually knowing about it belonging to McD, plus its non-EU trademarks) for a short period of time in 2019 until appeals are done.