Because otherwise their lack of voting would decide the election. ;-)
But, in all seriousness, every state most provide an election, it is just a quirk of the composition of electorate (and heavy gerrymandering, mostly by Republicans) that has lead to the small number of battleground states being the decider. That is because the other states (with the possible exception of Iowa, it seems) are set to very likely fall for either Harris or Trump based on careful analysis of many different factors.
Now, that is not to say that there could not be a surprise or two (looking again at Iowa), but the vast majority of states are effectively “locked” for one candidate or another based on the make up of the voters and the districts in those states.
The battleground states are where that is not the case—they could conceivably go either way. So they are the ones most closely watched and scrutinised by political junkies, pollsters, analysts, and operatives, and courted by candidates and campaigns.
And this is why, to paraphrase a recent comment by the French minister, we are in a position where a small number of voters in Wisconsin can have a large role in setting the security policy of Europe.