In a national campaign, anywhere you don't advertise, you leave up for grabs by your competition. Currently, candidates can get away with leaving most undecided voters on the table, because the mechanics of the electoral college mean that their votes don't matter anyway.
That fundamentally changes if I can counter your Times Square billboard with a direct mail campaign in Minnesota.
> In a national campaign, anywhere you don't advertise, you leave up for grabs by your competition.
And you can have Nebraska. The whole state, I wouldn't run a single ad there for a national campaign, in either scenario. Why would I? The ad money I might have to spend in Nebraska to get a thousand eyeballs might get me closer to ten thousand eyeballs in California or New York. Not to mention the ten thousand eyeballs I get in New York are also eyeballs I want to see my general platform and the fact that my party exists, because as you may well know, most people vote a straight ticket.
I see a lot of compelling reasons to believe that local media would remain a primary means of communication between candidates and prospective voters, even if we switched to a popular vote.
Wyoming and California have similar (when compared to the primary price driver) overheads in airing any given commercial, the price differences are mostly going to be market driven, and the 10k eyeballs in California won't end up costing as much as what you'd have to pay for 10k eyeballs in Wyoming.