"Because you're going to give me a job" = likely truth but bad answer...
Joking aside, really you should be able to answer this for yourself. However, decent answers could be: good/modern aircraft, reputation for having a good company culture, good progression prospects, great safety record, childhood dream (seriously; say you're applying to a world leader / big name and they'd probably not be unhappy to hear this), previous contact you've had with them has really impressed you, interesting route structure, compnay provides great service, you feel you could fit right in with these positive aspects etc etc.
In short - feed their strengths back to them, with enthusiasm, as long as you sound sincere.
What the above should tell you is that you can use this Qn cleverly to show them how much great stuff you know about their company.
Good luck.