If asked why Mbappe left PSG even though he was successful at PSG and his salary was also high. Of course this is an easy answer because Mbappe wants to win the Champions League!
Mbappe has achieved everything during his time at PSG, he has also won the World Cup with his country. The Champions League trophy is the only thing that is difficult to win with PSG, so he chose to leave all the luxuries that PSG offers including a high salary to get something that has never been won so far.
In addition, Mbappe is still very young and talented. Mbappe was also born in Paris, he has defended PSG for seven seasons. In Ligue 1 he has won everything several times. He also almost brought the Champions League trophy to Paris for the first time, but unfortunately they lost to Bayern Munich in the final. He has done everything for PSG to win the Champions League but has not paid off. At the international level, he managed to win the World Cup with France in 2018.
Moving to Real Madrid would allow him to develop faster because the level of popularity and competition in La Liga is better than Ligue 1, he also has a great chance of winning the Champions League with Real Madrid. Now that Mbappe has turned the page on his career, he can realise his dreams during his time at Real Madrid, including lifting the Champions League trophy and winning the Ballon D'or.