Many developers dream of their game being played by as many people as they possibly can convince. One of the things that they aspire to is having their game be played as an Esport (a game turned into a competitive sport) by professionals, fighting for a prize pool of sometimes even millions of dollars. Overwatch was one of the first games to try this concept on a wide scale with their "Overwatch League", and while it was successful for a time, it's also a good example of why every game does not need to be an Esport.
"When you try to appeal to everyone, you appeal to no one."
This is a phrase that is important to remember to understand the problem of trying to cater to Competitive players and Casual players at the same time.
Obviously, games try to do certain things differently than the others. Some games try to be survival focused, some try to be shooter focused, and most importantly to this post, some try to be competitive, and some try to be casual. Good game designers attempt to appeal to one or the other to garner a massive playerbase, however, that design philosophy has become increasingly uncommon, with many developers deciding to instead attempt to appeal to both, or they take a game that was originally made for a more casual audience, and try to morph it into a competitive experience.
Let's use Overwatch as an example once more. That game was focused on a casual audience at first with its many MMO mechanics and extremely small emphasis on the idea of the game being balanced around it being a competitive game people play for money. However, that changed a lot with its introduction of Overwatch League, causing them to often have to choose between retaining casual players or competitive players, which often ended up alienating both of them at the same time.
The two best examples of this are:
The addition of Role queues to kill the competitive strategy called "goats" (where the team would run 3 tanks and 3 supports), which helped the game's competitive side, along with preventing other team compositions such as having no support characters, but harmed its casual side by killing the amount of freedom they had to play whatever they wanted, and instead forced them to play a certain role.
Reworking Mercy, a character perceived to be overpowered by competitive players because of her resurrection ability, but was enjoyed by casual players because of her ease of use. The developers ended up nerfing her, and it ended up harming casuals.
There are more examples, but the point is when a game caters to both audiences, it caters to none of them.
One may try to argue that games based on rulesets such as Rocket League, Smash Brothers, Team Fortress 2, or others are able to sustain a competitive and casual audience, which is correct. All of these games are able to have competitive and casual coexistence by allowing the use of rulesets that can be altered and changed as much as the player wants.
Such as in Smash Brothers, it can be played as a competitive player and disable certain stages and items, or choose to be a casual player and allow items and all the stages to be played. However, not every game has this luxury and instead focuses on servers with pre-set rules (or they might have it as an option, but it isn't the main method of playing, nor is it used for general play.)
To be honest, there are many more examples of games attempting to cater to both audiences and losing players because of that. Online games tend to do that a lot in this day and age, far more than they used to.
For future reference in video game design, either cater to a competitive audience or a casual audience. Do not try to do both, or it will end up alienating both. This isn't just applicable to games, but most things in entertainment.
Role Queue explanation: https://overwatch.blizzard.com/en-gb/news/23060961/introducing-role-queue/