What are the major reasons behind for making the bitcoin transactions public for everyone to see?
The 'reason' is to have a fully decentralized and secured system.
It can only be truly decentralized if every participating actor can independently verify each transaction himself without needing to trust anyone.
Storing them in a public immutable ledger is a pretty elegant way to achieve that.
After Bitcoin hit the mainstream adoption by the people around the world, [...]
Bitcoin did - by far - not hit mainstream adoption yet.
Only a small percentage does know what bitcoin is. And an even lower percentage does own/use BTC's.
We are stil far away from mainstream adoption.
Because of that now people have to use the bitcoin mixing service to cover their tracks while using bitcoin for an anonymous transaction.
Bitcoin never was anonymous, and also never was intended to be so.
It is pseudonymous. And this usually should be enough for real-world-purposes.
For people who insist on having a better privacy, mixers are available. But noone HAS TO use a mixer.
Addresses can not be linked to real life identities. Only pseudonyms (the addresses) are visible.