This was a shower thought but I wanted to make a thread in case Oda decides to pull this troll move. 
There are multiple reasons:
1) People expect Mihawk to be #1, but while Mihawk is a powerhouse he has no crew and is not directly challenging the WG or conducting other illegal activities, thus decreasing is threat level and expected bounty.
2) While Buggy was considered a fodder (put in ID level 1) later on the marines found out about his past with Roger and his links with Shanks and actually started considering him a big shot (which is also why he got the warlord job later on).
https://mangasee123.com/read-online/One-Piece-chapter-549-page-10.html
https://mangasee123.com/read-online/One-Piece-chapter-549-page-11.html
3) Moreover, after MF Buggy assembled a crew (Buggy Delivery) that performs mercenary jobs around the world under the (then) protection of the warlord status. Having a crew and conducting illegal activities is going to boost his threat level further.
[To be fair, it seems that he's gonna ditch the crew while he tries to escape https://mangasee123.com/read-online/One-Piece-chapter-956-page-15.html ]
4) Buggy has long been a gag character and this would further reinforce that aspect.
There are multiple reasons:
1) People expect Mihawk to be #1, but while Mihawk is a powerhouse he has no crew and is not directly challenging the WG or conducting other illegal activities, thus decreasing is threat level and expected bounty.
2) While Buggy was considered a fodder (put in ID level 1) later on the marines found out about his past with Roger and his links with Shanks and actually started considering him a big shot (which is also why he got the warlord job later on).
https://mangasee123.com/read-online/One-Piece-chapter-549-page-10.html
https://mangasee123.com/read-online/One-Piece-chapter-549-page-11.html
3) Moreover, after MF Buggy assembled a crew (Buggy Delivery) that performs mercenary jobs around the world under the (then) protection of the warlord status. Having a crew and conducting illegal activities is going to boost his threat level further.
[To be fair, it seems that he's gonna ditch the crew while he tries to escape https://mangasee123.com/read-online/One-Piece-chapter-956-page-15.html ]
4) Buggy has long been a gag character and this would further reinforce that aspect.
and of course he got the Best luck in One piece his abilities unmatched
