Hello Heesob!
The main use of the tense 'Pretérito mais-que-perfeito' (pluperfect) is to talk about a previous past event to another also passed. BUT this tense has other uses.
And a really popular use of the 'Pretérito mais-que-perfeito' is to express a wish, a hope or ambition of achieving something (as Murillo said). So it's very common you hear expressions that use 'Quisera' and 'Quem dera' expressing a wish. Examples:
- Quem me dera estar no lugar do Paulo! (I wish I were in place of Paulo)
- Meu filho, bem quisera eu que tu estudasses mais! (My son, how I wish you studied more)
That's is why 'quisera' (pluperfect) was used in your example instead of conditional. The translation could be:
- Bem quisera, disse o principezinho, mas eu não tenho muito tempo. (I wish, said the little prince, but I do not have much time)