Dear Dildar,
Different factors contribute to decisions about GSoC, including assessment of the student's technical ability, interactions with him or her, and the interest of mentors in a particular topic. The accepted projects ranked highly in all those areas.
I noticed that many of the successful applicants (in this and previous years) had completed some initial work or proof of concept for their project topic. Such a track record helps the organization mentors be more confident of the applicant's interest and ability for a particular project. So, if you want to maximize your chances of acceptance for either Octave or another organization in a future GSoC, my recommendation would be to find a project idea that interests you and begin learning about it and contributing to it.