Because we suck when it comes to entertainment. Every kind of entertainment.
Our movies are 99% trash, our music is really bad, at least the mainstream music you see on TV or hear on the radio. The strange part is that the music that sells the most is 100% not the music you get to see and hear by mainstream media.

JBG 3 by Kollegah and Farid Bang just released and reached insane numbers on iTunes, Spotify and regular CD sales. Yet you won't hear them anytime soon on the radio. Same for some rock bands, even though they don't reach these sales figures.

Oh and our books suck as well. I prefer some thrillers from nordic countries over our books every day of the week.

Language. Simple as that. Also we had a huge Empire that straetched over 3/4 of the world.

Mercedes-Benz
BMW
Porsche
I say German popkulter is doing ausgezeichnet

And even a lot of people in China or Japan would not be considered English speakers but they went to school learning it for years. Also is that mandarin Chinese is a group of a bunch of different Chinese languages whilst English is usually just English with minor differences.

Germans have outsized influence in economic theory and classical music. Just because some Japanese games are popular doesn't mean Japan is more culturally relevant in the world.

You guys have the best Oktoberfest in the entire world, how is that not popular?

I see someone forgot about Uwe Boll.

Germany is famous for a lot of other stuff than just pop culture.
Inventions made by germany. People like Einstein, Nietzsche, Schopenhauer, and far more. We kicked the roman empires balls. Germany had always their prensence in time by being good or bad.
If it comes down to music and movies, there are certainly some gems there, but most of it is pure crap.

We have some really good childrens books by Cornelia Funke (the Inkheart trilogy), Ottfried Preußler (The little witch, Krabat), , Michael Ende (Neverending Story, Momo) and Erich Kästner (Emil and the Detectives, The flying classroom, Annaluise and Anton [<-- although her german name 'Pünktchen' would be much Closer to 'Dottie'), wich are internationally popular and some of wich have iconic film adaptations.

Foreign music crosses over to america very rarely anyways, but there's quite a few german bands and Artists that are popular all over europe.

As for german TV and cinema, a lot of it is just.....not very good. Germans like to be safe with their productions and it often hurts the content. A lot of the productions are samey, boring and predictable and whenever a good idea comes out it gets ripped off and bastardized to the point that you have trouble even enjoying the original good show.

I recently read an article about the german production landscape that went something like this: 'The US produces with money, the UK produces with guts and Germany produces with fear'. That hit the nail on the head pretty much.

We have some gems in there, but mostly German TV and film production just doesn't do anything you can't get from the US or UK.
Germans also made up a sugnificant part of early colonialized America and, in a way, jewish culture after the exodus of german jews during second WW and the holocaust, so a lot of german cuture is already part of american culture anyways. (Think christmas trees for example).

Add to that that any german production has to face an uphill battle internationally because of dubbing. The most prominent thing that crossed over from japanese culture are Manga, Anime and Sushi. The three things you absolutely cousln't get outside of japan, that were unique to that culture and that weren't emulatable.
Everything else gets remade, rather than dubbed, we don't get japanese TV shows airing straight up and most of the time we don't even get dubbed japanese movies. Even blockbusters like 'the ring' had to first be remade for western audiences before stuff like 'Uzumaki' could come over subbed.

So in a nutshell Germans just don't have a culture that is diffrent enough from american or english culture to be interesting on it's own merits and doesn't have production quality to counterbalance and overcome that.

We're a bit more low key. You have to be when you want to succeed this time.