In the Goblet of Fire, the other schools participating the Triwizard tournament used means of transport like flying carriages (Beauxbatons) and ships (Durmstrang) when they could have simply used Portkey?
There are two reasons for the two schools not to arrive in Portkey. The first is that the students needed a place to sleep and stay while they were at Hogwarts, and I think they the students of their respective schools would prefer staying with each other instead of with the students of Hogwarts. Also it provided a grand effect. The visiting schools want to make a first impression of themselves, just as the castle was also being spruced up for the first impression of the visitors. This way, I would think, takes only two to three hours with the assistance of magic, and it has the added benefits of room and outstanding an entrance.
In the Wizarding World articles about the Hogwarts Express, it is explained that before the train they used to travel to Hogwarts using Portkey and other methods but preferred the train because kids can lose the object and can get sick from the trip.
Also a portkey is usually a small object you can use to teleport a small group of people but I don´t think you can have more than ten people touching a small object at the same time so probably they prefer a method like Hogwarts express, that can take a lot of people.