As the article points out.
Voting in the U.S. is regulated at the state level, so if online voting were to be introduced, it wouldn't be a nationwide system as in Estonia, a country the size of Maryland with only 1.3 million people.
A key to the system's success in Estonia is citizens' wide acceptance of a digital identity and electronic chip-enabled ID card. Essentially a digital signature, the ID card is also used for checking out library books, paying bus fares, and even keeping track of medical data.
...
"For the United States, voting online is very problematic because (of) our lack of national ID cards, lack of some of the prerequisites that Estonia has implemented," Levine told The Associated Press.
In addition, there's the "fear of big government," Levine said. Americans, he said, "are afraid of the creation of a very large national database. We don't have that yet, and there's lot of resistance to it."
When the District of Columbia experimented with an online voting system in 2010, hackers broke in and changed votes to fictional characters.
Not to mention that not everyone in the US has access to a computer at this time. It's hard to compare a tiny country like Estonia, in both land mass and population, to a republic of vastly different states and regions.