were they bored by the way the game was originally played?Probably. They were in the third season. Shows change over time.
did they think they would get more ratings?Not necessarily, but since it would keep players on their toes and help avoid predictable outcomes, it probably kept viewers interested a little longer. Predictable outcomes spell death in television. That was why Fiji wasn't very well-liked. Earl was an obvious winner even in the pre-merge episodes. The Onions making F5 and Marcus winning would have made an equally boring season, and the ratings would have suffered as they did in Fiji. Yes, there probably would have been interesting, subtle dynamics among that alliance (as there was in Fiji's main alliance), but this is television, and subtleness really isn't what most people want to see. They want major shake-ups.
Or do they just want to try and get people so stop forming alliances?No, they want people to form more alliances. Think about it. Susie and Randy switched over to Kota and became members of the dominant alliance there, but they still had ties with old Fang members. That's where the interesting dynamics come in. Are they going to stay with their new alliance or flip back to their old one? Which is better for them in the game? Randy chose to ditch his old alliance while Susie flipped back over to them, and we can see that Susie made the better decision. Candice ditched the alliance she made on Aitu with Yul and Becky and went back (literally) to Raro. These are interesting storylines. Let's face it: On the seasons without tribe swaps (Borneo, Australia, Thailand, Palau, Exile Island), we've ended up with Pagongings or near-Pagongings. The show would not have lasted if every season was like that.
