OK, I'm not really stating he is the weakest, just saying his hype is not enough for the common opinion that he is in fact the strongest.
If we think about it, chances are that Shanks was the newest Yonko before BB, he's fairly young and both Big Mom and Kaido seem to be top tier for a while now. So let's do the hype on each one of them.
Kaido
- Strongest creature, I believe it was said that on a one-on-one fight Kaido always wins
- Huge crew with according to the minks endless ships and over 500 Smileys
- Ridiculous hype on the 3 Calamities
- Immortal
- Rules a country full of Samurai
- Rio Polenglyph
Big MOM
- Highly influencial, seems to have a huge info network, and countless aliances based on her family
- Owns a Nation full of the most variate races and most likely powers
- Weird fairy tale power, no daught it will be something quite rare
- Likely the oldest Yonko
- Gonna be hyped beyond mesure through the new arc
- Rio Polenglyph
Shanks
- Strong crew, both BB and Akainu refrained from clashing with him at Marineford
- Good chances he has the strongest haki ever showed
- Swordsman skills were enough to doodle with Mihawk
- Crazy phisycal strenght, split a cloud once
BB
- 2 of the strongest DFs on him
- 10 titanic captains
- Seems to know how to steal and pass Dfs around
- D
- 2 major wins against Revolutionaries and WB's leftovers
- Bound to be one of the strongest characters by the end of the series
The point is Shanks is strong, but a beat up BB and Akainu steping aside is something any Yonko would cause at the time. His crew always shows up together and on a single ship, plus not a single pirate crew was showed to work under them, might be less influencial then the others. He might have the strongest haki, maybe. But so far nothing unique was showed, he's not even the strongest swordsman. Might take newbie BB, but he pales in comparison to the other Yonko.