Mere access to navigable water doesn't create a major port. Every place along the shores of the Great Lakes has a roughly identical geographic situation.
Erie does have a shoreline protected by Presque Isle from the open waters of the lake, so it has an advantage over most places fronting the Great Lakes, and that might have made it an important 19th century port funneling the trade of Pennsylvania—had the American colonies become independent nation-states rather than a federation with unrestricted interstate travel.
Cleveland, Buffalo, Detroit, and Toronto, on the other hand, developed as entrepôts, places where goods were being transferred from one mode of transport to another. In the case of Cleveland and Buffalo, this was because they were where canals met the lakes; Detroit and Toronto were the focus of wagon and later railroads for reasons partly due to geography (Detroit controlling a strait) and partly due to history (early settlements with significant defense investments). In some sense, this is a chicken-and-egg game: did early roads from the interior of Michigan or Ontario lead to those places because they were really the best location for a city, or merely because other roads already did? At any rate, some settlements flourish and others don't; like seedling trees, the ones that flourish get more and more of the sunlight.
Why is being an entrepôt important? Because as long as you're having expensive labor move goods from one transport mode to another, you may as well set up a factory or processing facility that adds value to those goods. So factories turn wood into windows or furniture; warehouses store big shipments of various foodstuffs and break it down into shipments the right size for a frontier grocery store; grain elevators aggregate the harvests of thousands of inland farmers and resell them in bulk. With enough of those small enterprises, plus the artisans and clerks, accountants and lawyers to serve them—a settlement becomes a city.
If I had to venture a guess at this it would have moreso to do with Erie's lack of jobs from the automotive section.
If we look at cities like Cleveland and Detroit most hit their peak in population during the period between the 1930s and the 1960s.
Cleveland had the two Cleveland Engine Plants open that decade that brought alot of jobs into the area and at its peak employed more than 15,000 employees and turned out 900 engines per day. This plant is where some of the more iconic Mustang engines were built; however, Ford has since shrank production there by closing one of the two engine plants.
Detroits peak employment was even larger with some plants, like Ford's Rouge River facility, at its peak that plant alone employed over 100,000 people.
The jobs brought on by automotive manufacturing and the plants drove the population in Detroit and Cleveland; hence, why we are where we are now. The plants closed, the jobs left, and the cities populations began to decline.
Cleveland, as of 2016, has an estimated population of around 365,000 people.
Detroit, as of 2016, has a population of around 672,000.
Erie, PA simply didn't have these kind of jobs; hence why it never saw the same boom.