What I don't understand is the bull season is caused by Bitcoin halving, so why do people are looking to invest in other projects when it's clear Bitcoin is the coin that the price will increase?

this applies to Litecoin halving, people should buy Litecoin, Ethereum upgraded to Ethereum 2.0, people should buy Ethereum.
Most people believe in diversification and that is why they buying other coins after the halving. And another thing I also think of is the All Time High before the halving. Bitcoin hit 73k$ before the halving and the prediction was bitcoin would faced some elements of bearish before and the halving period but the opposite was the case and after the halving the price of bitcoin started to decline and that made many people to started to invest in other altcoins thinking that they would do well in the market but irony was the case again.
It's a bad advice to rely your financial thing with other people, they will not be responsible with their false predictions.
In cryptocurrency we rely on prediction and in the process of the observation, there are some prediction become false while some are accurate. We can't neglect prediction in the cryptocurrency market.