Tanev and Brodie are a wash basically, Zadorov for Niemela+Timmins probably doesn't happen. How is CGY adding a 1st?

Agree, this is the type of defenders the Leafs need. BUT this is a very one sided trade (for the Leafs). The Draft pick is going the wrong way. Even if you took the 1st out Calgary would decline this.
The Flames would not trade both Tavev and Zadorov (especially if Hanifin is on the way out too). If they held these players to the deadline, they could get a late first for each player from a contender if they retained salary.
The only asset that has real value from Toronto is Niemela (A-Prospect). Brodie's play declined last year, especially in the playoffs so this is a cap dump.
Tanev is one of the best pure defenders in the NHL and Big Defenseman like Zadorov are going for a premium right now. Leafs would need to add a lot more...

Tavev is one of the best pure defenders in the league. He is a much better player than Brodie (who really started to decline last year). This is not a wash. If you gave teams around the league the choice between the two, 95% would take Tavev and he is $500k cheaper. This is why the Flames let Brodie leave and signed Tavev.

Brodie was unhealthy for a good chunk of last year which could explain his decline. He's not Tanev who is always getting injured and probably playing through something.

What exactly is the reason thr Flames let Brodie walk and signed Tanev instead?

Everything you stated happened after that and I would say they signed Tanev because he made a good replacement since they are very similar. Tanev just eats a lot more pucks which is part of why he's the better shutdown defenceman.

No way does Calgary do this.