Because, and pay attention here because THIS IS VERY COMMON
It is easy to count votes in places where not many people live, and it takes a long time to count votes in places where a lot of people live.
And we already know that Republicans do very well in places where not many people live, and Democrats do very well in places where a lot of people live.
So once you've counted the votes in the suburbs where the real swing votes are, you can pretty confidently (95% accurately) predict how the rest of the votes are going to go.
Which leads to what you're seeing which is called a Red Mirage AND IT HAPPENS IN EVERY ELECTION SO GET USED TO IT: Republicans look like they're winning because their votes in the countryside are counted very quickly but the city votes where the numbers more than make up for it can take all night.