Why am I not eligible for a scholarship at UCSD?
It says that based on need, I could get around a 50k discount, but I'm not eligible for any scholarships. Why am I not eligible for scholarships but am able to get a high discount?
There are two different kinds of scholarships that most schools will offer. Merit scholarships are the type that you apply for based on your academics, extracurriculars, and other things that you've accomplished or done. There will also sometimes be merit scholarships available based on background—for people from certain locations, cities, people of certain backgrounds, or people from even specific families.
There is also need-based aid, which is the "discount" you're mentioning. Some schools will calculate what they think a student's family can afford to pay to attend and be able to offer an amount in need-based aid (often in "grants," which are free money that you do not need to pay back) that sometimes even covers all the way up to that discount point. Some schools, e.g. Harvard, can use need-based aid to make themselves effectively free for students below a certain income threshold (for Harvard, they're free for families making less than $65,000/yr, and families making up to $150,000 usually are asked to pay about 10% of their income). I'm not clear from your question whether UCSD has actually offered you this need-based aid as an accepted student or if you've just looked at the calculator on their website and seen that as your number, so if you could clarify, that would be helpful in answering your question.