* Germany was at the frontline in the cold war. So Germany is traditionally both a target and a host of US spy activities.
* Germany hosts important US military facilities: US nuclear bombs, US central commands for Europe and Africa, military airports, military logistics, military hospitals.
* Germany was reunited with a communist part, which was under the influence of the soviet union. Thus it hosts a lot of former communists, former Stasi (East German State security, ...), a lot of KGB contacts, etc.
* from Germany it is easy to spy on East Europe
* Germany has a successful industry which is a great target. Germany has about 1500 so-called hidden champions, SMEs active in (often high-tech) niche markets and dominating them. Germany has SAP, one of the largest software companies world-wide, whose software is used at the core of the business of most of the large companies world-wide.
* Germany is a part of the EU, something which the US likes to understand and influence
* Germany has no direct border controls to neighbor countries, so there is a lot of illegal immigration to Germany, which makes it interesting for the US
* Germany is very active in high-tech and especially the Internet. Germany hosts one of the most important Internet exchange nodes.
* Germany has deep trade and diplomatic relations with Russia and China.
* Germany hosts some international relevant institutions, above all the European Central Bank.
* Germany has a relatively large military industry and exports military goods (tanks, airplanes, submarines, ships, satellites, ...).
* Germany has active left and green parties. Suspicious!
Plus Germany lost WWII and had to give up rights, some of them the US still has - so it may be still legal for the US to spy in Germany.
</sarcasm>