The Electoral College offers sparsely populated states a disproportionate amount of influence over the electoral process. This influence is magnified particularly in swing states. In the 2016 general election, about two-thirds of campaign stops were to the 4 biggest swing states: Florida, Ohio, Pennsylvania, and North Carolina.
This means that candidates are not campaigning across the nation. Additionally, this makes some voters in states that are considered 'safe' or electoral strongholds for a particular party under the Electoral College feel that they are dis-incentivized to vote, as their votes do not have equal weighting compared to swing states.
Finally, if we had a popular vote election, candidates would still not be able to focus solely on the large states because there aren’t enough votes there. In 2016, the 4 largest states (population-wise): New York, California, Texas, and Florida only had about a quarter of the total votes. This means that even if a candidate got every single vote in those states (which wouldn’t happen) the candidate would still have to be campaigning elsewhere.