Why do I have to pay back all of my healthcare advance subsidy if my income was less than $100,000?

if line 5 of form 8962 is more than 401% of federal poverty line (see below) you aren't entitled to any PTC.  

other than Alaska and hawaii
federal poverty line the first number is the number of family members
1 $12,140
2 $16,460
3 $20,780
4 $25,100
5 $29,420
6 $33,740
7 $38,060
8 $42,380