Because in 1/3 of the elections this century, the winner of the popular vote did not win the Presidency.
It is misleading to evaluate a system based on 20 years of a 232 year history. In those 232 years the electoral college winner did not win the popular vote in 5 elections (in 1824 no candidate won a majority of electoral votes and in 1876 a special commission made the decision). "This century" is only 1/5 complete.
It's not a competitive system if one party has to rely on rigging the rules for its electoral benefit because it can't legitimately win an election, and is openly contemptuous of the democratic process.
No party "rigged" the electoral college system for its benefit since it was part of the Constitution before the current parties even existed. A Constitution which made no provision for popular votes.
But WHY would one party dominate? Because their policies appeal to the vast majority of voters. You're arguing against that. If you want to win elections, you need to get more votes. If you can't get more votes, why should the system give you a handicap? Because you think you're entitled to it? You're not.
I don't think one party would dominate. That was the assumption made in the OP. Both candidates got a record number of popular votes in 2016 showing the close competition between the parties (especially when you consider the congressional and state races).
The system gives neither party a handicap although changes in party and regional loyalty change the advantages of each party over time. Republicans benefit by the population movement to red states and will gain more House seats and electoral votes by 2024. Democrats benefit by demographic changes. And, of course, coming up with a popular candidate determines a lot of the outcome since policies are less of a factor today than group loyalty.