For me the reasons are favoritism and being similar to the main characters of the Fate series.
1 - Kama and Parvati: The main reason behind my choice of Kama and Parvati is because of how similar they are to Sakura Matou, and since I just watched Fate/Stay Night and Fate/Zero 2 years ago.
These were one of the reasons I decided to choose Kama and Parvati because after I saw this scene from the first episode of the Fate/Zero series I was like: My god, Sakura Matou deserved a better childhood, a better life.
The 2nd reason made me want to teach this old man a lesson to learn not to ruin Sakura Matou's childhood, at least I managed to do that in Paper Moon when Zouken appeared.
2 - Illyasviel von Einzbern: The reason I chose Illya was in the year 2020, because it was the first Fate Kaleid series I started watching, but apparently I managed to finish the 3 seasons + 1 movie of Fate/Kaleid 2 years ago, it was because of the feeling of favoritism in relation to Illya.
When I started playing FGO 2 years ago I never noticed that Illya was summonable when I was exploring the Fandom Wiki, when I found out about Illya being summonable in FGO, so I decided to wait until August 2023, which resulted in me getting Illya Caster + 1 copy, Miyu and Illya Summer, and I also got spook which was Heracles.