TibiaBR's owner was not pleased with some of the rules CipSoft required fansites to abide to, specially concerning what they were allowed to advertise in the fansite or not. Particulary, TibiaBR wanted to publish ads of a Tibia Coins gambling website which CipSoft did not allow. This was the main reason why TibiaBR decided to leave the program, as they decided it would be more benefitial for them to have the freedom of advertising anything over the perks of being a supported/promoted fansite.
After they left the program, they also started to invest on their own OT server as it would no longer be a problem for them.
The fansite's popularity itself was already in decline, since it was heavily focused on translating news and forum-based content. Since automatic translations became better and more popular over the years and forum content less popular (since most content creators moved to Wikis, social networks, youtube, twitch, etc), TibiaBR was already short on staff and struggling to create content of its own. Without the space to promote content on official News Tickers and create contests with special prizes, over the years it was indeed left aside by the community in favor of other sources of content.
Finally, it's worth noting that the owner of TibiaBR is also the owner of other fansites such as Portal Tibia and the Brazilian Tibia Wiki, which still share some of the staff with TibiaBR.
Source: I was also part of the Staff of TibiaBR for a couple years and saw this process happen.
Edit: After the fansite left the programme the admin recorded a video where he explain his reasons behind the decision, which I realized is where I got most of my story from as well. You can search it on youtube by "Por que o TibiaBR deixou o programa de fansites oficiais?". It's obviously only in Portuguese.