A few things and of course I'm relying on stereotypes here:
1. Men's clothing is so much cheaper to begin with that men may be less likely to shop at thrift stores so there's less motivation to provide stock for them.
2. Men have smaller wardrobes so their clothing gets worn harder during the time that they have it and so be less suitable for donation.
3. Women's clothing is expected to fit more snugly so they outgrow it quickly with any weight gain at all while men can buy a shirt when it's baggy and "grow into it" until the buttons are about to burst, so, again, less and slower turnover that can be donated.
4. And I think this is most important, women's styles change quickly while men can hang onto the same clothing until it falls apart. (So if it's any consolation women looking for classic styles tend to have a difficult time clothes shopping too.)
5. I'm reaching here, but maybe due to the fact that men are stuck with rather boring clothing, that is to say, a very narrow acceptable range, thrift store shopping may not be very rewarding for them while women can always hope to find something interesting, so, again from point number one, lower customer base results in less motivation to carry inventory for them.
I don't know Germany at all but I do know in the US that thrift store availability and inventory can change a lot from town to town and neighborhood to neighborhood. While poor neighborhoods are more likely to have a thrift store, if you can find a rich neighborhood with one, their selection will be much better.
Others mentioned consignment shops and you might see if there's something like that there if you're looking for professional wear.