Because the Church is in Communion, and thus deeply one, it can afford to have different approved externals in matters of custom.
Heck, certain Latin Rite religious orders (like the Dominicans) used to have slightly different Masses from those found elsewhere. The Church okayed this because it found that kind of diversity valuable. Similarly, the Church doesn’t say that everybody has to get married, or everybody has to be single, or everybody has to vow poverty, or that every religious order has to have the same rules.
The Vatican is not going to tell ancient Christian communities that fought for faith against heresy and persecution, for centuries, that the things their saintly forebears followed were all wrong. (This sort of thing has been done, but it usually becomes obvious pretty fast that it was a mistake. It also usually makes a huge mess that takes decades or centuries to clean up.)
It would be presumptuous for Peter to impose burdens instead of strengthening the brethren, especially when Peter thinks a lot of the ancient Eastern customs are extremely nifty and reverent, and have a lot to teach us about Mass and Christ and the Church. But it would be equally wrong to impose Eastern customs on the West. You don’t transplant a tropical flower into snow.
So instead, we have several rites, with several different ways to worship God, each approved by long custom and the Holy Spirit, each in communion with the See of Peter. Each bishop also has certain areas in which he can lay out his own rules for his own diocese, and each pastor has certain rules in his own parish. All parents have the right, within certain bounds, to lay out rules for their own households, too. This is called the principle of subsidiarity.
This may seem strange, but most people live it every day in the secular world. The federal government of the US has certain rules, but every state and every city or county have their own rules, too. Some people can buy liquor on Sunday, some people can only buy during the week, and some counties can’t even sell you weak beer. Is that unfair? No; it’s more fair than having one rigid law code for all. Within bounds, they can have the law that suits them.