First concept: playstyle. There are only three pure playstyles: martial, magic, skill. The quintessential classes are fighter, magic-user (or wizard), and thief (or rogue). Every other class is a hybrid or variation. Paladins, barbarians, and rangers were once variations or subclasses of fighter.
Second concept: specialties. What makes you special is that you can do something that no one else can do. When you take something special about one class and make it available to others, it is no longer a specialty and is trivial. For example, fighters were once the only class who could regularly hit for consistent damage. Now everyone has a way of doing consistent damage such as cantrips. Wilderness survival skills such as tracking were once unique to rangers.
Third concept: niche or identity. What is a ranger? A ranger is a character who defends civilization by fighting the evils that threaten it on the boarders of the wilderness. They are in essence a fighter who has traded heavy armor for wilderness survival skills. This is easily stolen from them by the Survival skill becoming available to other classes and by a lack of emphasis on the exploration phase of the game. A Rogue can put expertise into survival and out survival the ranger.
To summarize, Rangers were once a fighter subclass that is no longer special and they're part of a class roster in a game that underemphasizes exploration which is their brand.
What has modern WotC done for the ranger? Made the ranger a combination martial, magic, skill character, which we have plenty of, with features most players won't use such as nullifying difficult terrain when hexcrawling, or features that aren't as valuable as another feature from another class, like how favored enemy is not as good as sneak attack. Animal companion is not a worthwhile feature imo because why couldn't anyone have an animal companion when bloodhounds and game hawks ought to be a commodity you can buy from a trainer? Hunters mark? Why not just give the ranger bonus damage die on all attacks since they'll be using it constantly, and so it will simplify the Ranger player's turn? Call it a built-in hunter's mark damage die.
To fix the problem, you need to take the ranger's things away from the other classes and give them back to the ranger. It's like giving anyone else unarmored defense, battle rage or an inspiring voice that gives bonuses orcsneak attack. And they have to put in some really good exploration rules and instructions so people understand hexcrawling or point crawling and want to use these rules. I've had 5 GMs now, and none of them understand hexcrawling. They all handwave it. Even Matthew Mercer, celebrity GM, doesn't do hex crawling. Maybe hexcrawling just isn't that fun? Alternatively, make ranger a subclass of fighter or rogue.
