Spidey we don't have any info on why Bianco slid as far as he did but we do know there are enough examples of players sliding well past their expected places in the draft who become fine players. Contrary wise there a enough examples of high expectations unrewarded to convince me at least that picking players from the national draft is far, FAR, FAAARRR from the science many make it out to be.I wouldn't say either examples are comparable.
As has been mentioned the likely hood of a Swanny dropping that far is much more unlikely these days. Recruiters have a much better grasp of top end talent now days. Plus Swanny had many attitude questions about him. For every Swan there's a Dayle Garlett.
Grundy slid because the vibe at the time was it's not worth risking a first round pick on a ruck. Better to trade for an established ruck. By geebus we made out like bandits though with that poor theory that abounded at the time.
If the same "theory" was applied this year I'd be shocked if Luke Jackson went in the top 20.
It's an art that the best recruiters have a feel for and cannot be acquired by any means unless gifted at birth.
The why is unimportant to the result.