It averages less money to deliver to clusters than to curbside, and less to curbside than door-to-door, and the savings, figured over the hundred million plus deliverable addresses, is substantial. I'll give NPR some page hits, but this is sample coverage -- you can find comparable articles ... everywhere.
"About 30 million residential addresses receive delivery to boxes at the door or a mail slot. Another 87 million residential addresses receive curbside or cluster box delivery.
The cost differences are clear. Curbside delivery costs average $224 per year for each address, while cluster box delivery averages $160. Door-to-door delivery costs the agency about $350 per year, on average."
There are probably still places that could transition from door-to-door to curbside (city neighborhoods of detached, single family homes, assuming that any of these retain door-to-door service), however, densely populated urban neighborhoods would be hard pressed to find space on sidewalks to place cluster boxes, a point which is made in some articles.
Alas, no one seems to have put together another problem with cluster boxes: theft.
As with ATMs, thieves occasionally resort to just taking the entire cluster, altho more typically they pry open the large door used by the carrier and then distribute the contents they aren't interested in on the landscaping nearby. As with other mail theft, this often occurs around Xmas as thieves are looking for monetizable items (gift cards, washable checks, etc.).
Realistically, it seems extremely unlikely that the USPS will be allowed to do any innovating on this scale, even if they wanted to (and it doesn't sound like they do). It also seems unlikely that both houses of Congress will agree on anything. In particular, moving to door-to-door to cluster boxes seems likely to piss off a lot of people who probably vote Republican, and reducing the number of carriers needed will certainly piss off the postal unions and thus be unpopular with Democrats.
Besides. The pension fund is massively overfunded, so they should just quit making payments on that anyway.
ETA: From last year's version of this discussion.
ETAYA: Conversion from door to curbside already occurring. Slowly.
ETA still more: If anyone can find a copy of the Modes of Delivery report, I'm dying to read it. Alas, the link at USPS is broken and the Internet Archive does not have a copy. :(