Does he actually (that we know of) have a link to the Fetts? Or did he just like the helmet?
As I understand it, there’s two ways of looking at it.
If you want to suspend your disbelief to enjoy the series then it’s:
Mandalorian’s are a Humaniod race known for being ‘ing nails.
Fett was a Madalorian and wore the armour of the Madalorian’s but he’s not the titular Madalorian in ‘The Madalorian’.
If you want to know the slightly more boring real-world answer, they’ve been trying to make a live action Star Wars TV series for years. After the Prequels were made they were going TV series which was going to explore all the non-Jedi v Sith / Rebellion v Empire bits of the Star Wars ‘Universe’. At first it was going to be about some lessor characters advenutres between the prequels and Original Series, Solo Origin, Lando etc, but this was scraped in favour of a series called Star Wars: Underworld which was going to be about Bounty Hunters, the Huts and the other criminal elements of Star Wars. That got scrapped too, this was pre-Game of Thrones so it wasn’t believed you could make a multi-million dollar an episode gritty fantasy drama profitably and 10 years ago CGI etc was more expensive to produce than now.
Things changed when Disney bought Star Wars, they invested massively into it with new films to grow the franchise for a new market which made an expensive TV series more likely to be profitable, add to the fact they have Disney+ to launch and some of it’s massive marketing budget for that can be used to make this.
In the great tradition of ‘Hollywood’ no good ideas (or indeed bad ones) are ever wasted, some of the bits of the TV series went into ‘Solo’ and Rouge One, lots went into the Animated Series but the core of Star Wars: Underworld have been evolved into The Madalorian, they’ve kept enough of Fett to grab interest, but not so much that viewers might not be able to find a hero one of the Baddies of the original films.
There’s 2 more TV series coming, one based around one of the characters from Rouge One and the big one about Obi-Wan Kenobi staring Ewan McGregor.