Roobet.com may not allow people from Germany to use their platform for reason like there are specific rules and licenses required for online gambling in Germany and Roobet.com might not meet these requirements or want to deal with the legal. Roobet.com may be concerned about taxes and fees associated with operating in Germany.
But the best thing to do is check Roobet.com's terms and conditions to understand why they might block users from Germany.
There are plenty of nations that Roobet can't legally operate in, this doesn't mean that people from those nations can't play there, it just means that they can't legally allow you to gamble there, but if you want to gamble then you must take that risk yourself personally, and not expect them to be part of it.
This causes issues with the gamblers, like for example they may end up doing it from Germany or from any of the other nations that are not allowed to gamble here, and then when they get caught and their account frozen, they are going to end up complaining here that they were allowed to gamble when they were losing and frozen when they won, well that's normal they can't control EVERYONE, so they check the big wins only, because that means something, and if you get a big win then you will be checked too, and if found out that you are gambling from a nation that is not allowed, your account will be frozen. I warn people that they should be careful about that.