Well, Tesla is building its Gigafactory on the planned, but cancelled site of a BMW factory in Brandenburg.
- Lots of space already available for a large factory
- Much more space available for expansion
- Building permit is thus easily obtained, they just need to copy the old permit for BMW and put Tesla in its place
- Right next to the Autobahn A10
- Commuter trains from Berlin end just a few hundred meters away
- Large pool of skilled employees
- Many universities around, good R&D culture around Berlin, popular among Startups
- Many Tesla suppliers are already in the neighborhood or operate not far away
- Many other large engineering related companies produce next door, like Bombardier or Rolls Royce
- International Airport only some distance away and will be much closer should the BER ever open
- German government doubled the subsidy for electric cars just a few weeks ago (Making the ID.3 of Volkswagen just as expensive as a similar VW Golf 8, the Model 3 is in the same price range right now)
- A good central site for the planned R&D center for Tesla in Europe is also already proposed inside Berlin Marzahn, additionally to the estate in Brandenburg
- And of course, many Tesla managers are German, so the German culture is also not alien to them
- And since we are expecting a worldwide economic recession in the next years, the reliable but boring Germany is much more interesting than the more exciting places for libertarian economists.
And of course the killer argument in case of the UK: A very professional and reliable government on federal and state level. The negotiations happened very silently despite a state election during that time, no involved politician used the possible Tesla deal as election argument or promise. Musk especially mentioned that yesterday.
Of course the rather poor state of Brandenburg badly wants large industry and will likely help Tesla there as much as German and EU law will allow and WTO rules tolerate.
From what I can tell from the Volkswagen tribe, many there feel good about Tesla going to Germany since it gives them a lot of confidence that their choice of going towards full electric in the future and investing billions in developing the MEB (Modulares Elektrisches Baukastensystem, modular electric construction kit) for its future cars was the right decision despite many other German car manufacturers calling them nuts. Looks like Tesla also has faith in the European market.
And at least Volkswagen seems to some sort of admire Tesla just like it is the other way around: VW started to construct cars similar to what Tesla does (ID.3 and Golf 8 are both more similar to a Tesla technically, but with the full power of the VW supplier chain), while Tesla tried to become much more German in terms of production planning (they even bought one of the most advanced German companies for that strategy)