Lots of electoral votes. PA, OH, FL. A good trifecta needed for a Republican to win.
It is because it is close. You could say the same thing about Florida.
Heres one way to look at it:
If you assign all the states Trump won decisively in 2016 to Trump, he would have a base of about 225. He needs 270 to win. He won Florida by a couple points - so lets add another 29 and that is 254 - now he only needs 16.
Pennsylvania is 20, so that could put him over the top, as it did in 2016.
But a lot of people think that PA, MI, WI have a lot in common - so if it starts to look like Trump will win PA, that means hes more than likely to win those too.
Thats why a lot of us will be watching PA closely - if he wins Florida and then PA, Biden has no chance.