Cause pre time skip, Coby was even less of a threat for Luffy, than Usopp and now his current rank within the Marine is just a captain and he hasn't even accomplished anything, except for doing something in the Rocky port incident.
Meanwhile Hancock is a well know infamous pirate, which has been part of the three great powers.
With her hax devil fruit powers, she easily subdued a experienced Vice Admiral and even Sengoku called her strong, also she is capable of using all three types of haki and it's implied that she's a advanced CoA user on top.
Coby's growth rate would have to be even better than Luffy's, to go from being a one shot material for base pre time skip Luffy, to be above someone who fodderized Pacifista with ease, which base pre time skip Luffy was no match for.
He would be in the same leauge as Luffy before he absolved his hardcore training in Udon and fought Katakuri, even though pre- ttime skip, Coby was vastly below Luffy....