Terror vs. Islamist Terror
The cited source provides a statistic of terrorist attacks and not of Islamist terrorist attacks. Not all attacks were commited by Islamists. For example, Italy has frequently suffered terrorist attacks by anarchists, the extreme political left and also the Mafia.
I haven't found any explanation for the high number of terrorist attacks in Italy in 2019. However, I have found the following: In 2019 there were 118 terrorist attacks in the EU, including failed attempts or those prevented by the police. Among these only 21 were attributed to Jihadist terrorism (source (in Italian)).
Thus, the assumption by the OP is invalid. It's the location of these 21 attacks which matters. Did Germany experience less attacks than others? Is a statistic based on 21 observations still significant? My guess: one would have to include attacks from more than one year.
As stated by Italian philosopers 4 Monica in his answer: The country of origin of the Muslim population is different. The Muslim population in Germany has mostly immigrated to Germany under gastarbeiter status from Turkey, to a lesser extent from Maghreb countries. The most recent wave of immigrants has arrived from Syria fleeing from war.
Neither the Turkish immigrants nor the Syrians, who are more secular and grateful to have found a secure refuge in Germany, are as easily radicalized to an extent to commit a terrorist attack.
German Role in the Middle East
Usually Islamist terror is reduced to pure religious fanatism. However, many terrorists claim to (also) have political motivations.
The United Kingdom and France, as well as Italy (Libya) and Spain (Northern Morocco), had been colonial powers in the Arab-speaking world. They dominated and exerted direct control over much of the Middle East and Northern Africa leading to resentment from the Arab population. In particular, the secret Sykes-Picot-agreement between the UK and France persists as a major historical grievance. The creation of Israel (though opposed by the UK) is another one.
Germany was largely absent or had even been standing on the other side. During the First World War Germany was allied with the Ottoman Empire and fighting both France and the UK. In the Second World War many Egyptians were seeing Rommel's North Africa Corps as potential liberators from the British. Germany was hoping for a Muslim uprising against the British across much of its Empire.
Furthermore, many political extremists in the Middle East admire Hitler and wish to emulate the Holocaust. It doesn't seem to matter that Germany insists on Israel's right to exist and is a major arms supplier to Israel.
German role since 1991:
- First Gulf War: Germany didn't send any soldiers (and wasn't expected to do so). However, it paid a lot of money for the liberation of Kuwait. This is relevant as the presence of American troops in Saudi Arabia caused Osama bin Laden to turn against the USA.
- Afghanistan: Germany participated as much as any other NATO member after the American invasion. In the first few years the situation was relatively calm in the (non-Pashtun) part of the country, where German forces were operating. Reconstruction seemed to be on a good way. Later the Taliban gained in influence, though, and the situation became "less peaceful".
- Second Gulf War: Germany and France publicly opposed the war.
- Libya: Germany didn't support a foreign intervention to remove Gadhafi desired by France's Nicolas Sarkozy - who is said to have received money from him for his re-election -, Italy's Silvio Berlusconi - who had honored him by letting him erect a huge tent in the center of Rome for his visit -, the UK and to a lesser extent the USA