Simple, lighter weight. The builder, James Betts, had built several boats in this method before.
You mentioned guy elsewhere on this site, who spent a fortune on a carbon fibre mast. After extensive cruising, he said the improvement was minimal, almost unnoticeable . What was the cost differential between the composite decks and the aluminiumn one? What was the price tag on the composite one?
Aluminum has the advantage of letting you weld most of your deck hardware down, eliminating any chance of deck leaks. A USanian in a plastic boat, a week or so ago, told me he has to re-bed most of his deck hardware every few years, to stop them from leaking . He said the strictest adherence to the directions hasn't solved the recurring problem. Given that substantial weight savings high up in a mast has given minimal improvement , how would such savings much lower down, at deck level, be enough to justify the expense, and deck leaks ?
This is the kind of blind worshipping of so called "High tech", which keeps so many boats in a marina for most of their lives, while the more practical, realistic and enlightened enjoy far emptier anchorages than would other wise be the case.
I guess I should be thanking the pedlars of high tech, for keeping my favorite anchorages free from over crowding, by the gullible deciples of the "Consumerism " religion.