The Roosters have a winning culture, they are nearly always there or thereabouts. Joining the Roosters was a no brainer, his other option was to remain with a bog ordinary team.
While roosters are more likely to win a premiership it isn't exactly a guarantee.
I think I quoted Shaun Johnson from the warriors earlier, he said he could easily move to an Australian team and win a premiership, but it wouldn't have the same feeling as winning a premiership with the warriors, the team he came through the systems with. You could say the same about Tedesco
Tedesco could have led the Tigers to premiership glory, and became a one club legend. however now he will just be that bloke who went on to play for the Roosters for more money
Winning a premiership with your junior club is way more of an achievement than winning it with a club that has virtually brought their way to a premiership