I can't remember which book I was reading, but the author mentioned a particular type of freight bogie which appeared in the 1980s which was considerably cheaper than previous options and led to a shift towards more bogie wagons being built. I think they were talking about private owner wagons, but the same issue of cost would have applied to BR built wagons.
Given tight budgets and investment constraints, if there was a big cost difference it would have been a major factor in the type of wagons built, not something that can be dismissed as "penny pinching."
As mentioned already, there were also infrastructure constraints in various terminals built to handle standard wagon lengths of ~21 feet. I remember reading that one of the terminals handling grain at Birkenhead necessitated the use of 2-axle hoppers (instead of the larger Polybulk types) due to the curvature on the dock lines. There was a similar issue with many coal loading locations; even into the 2000s there were some locations unable to take bogie wagons. In the 1970s / 1980s Speedlink era it would probably have been difficult to standardise on a fleet of bogie wagons for general merchandise traffic. Even the long-wheelbase 2-axle wagons being built apparently caused problems at some locations, resulting in the VEA type, "
traditional wagon designs refurbished and fitted with air brakes in the early 1980s to suit customers who could not handle the longer wheelbase of newer designs" according to this profile on the excellent LTSV wagons website -
https://www.ltsv.com/w_profile_028.php
This picture gives a good illustration of the type of ancient sidings in use in places like docks (03 170 at Spillers Flour Mill, Wallasey Dock Rd, Birkenhead Docks). -
https://www.flickr.com/photos/36034969@N08/4891699868/
There's a huge size difference between the 2-axle PAAs and the more modern Polybulks, captured well in this view with 03170 again -
https://www.flickr.com/photos/feversham/12769190874/
The first bogie ferry vans to appear in the UK were also fairly massive compared to existing designs -
https://paulbartlett.zenfolio.com/german3doorvan
I think these ferrywagons were some of the first large bogie vans to appear in the UK. By the late 70s when they appeared, many of the BR long-wheelbase opens and vans had already been built.
The four wheelers built for bulk flows are more of a puzzle. The HAAs possibly needed to be backwards compatible with older mine and power station facilities, although of course they were designed to use new facilities with low speed loading unloading. Quite why things like cement tanks and aggregate hoppers were built as four wheelers I don't know.
This interesting article on private owner wagons -
http://igg.org.uk/rail/4-rstock/modpo.htm - mentions one reason for cement tanks being 2-axle:
In the 1970's Blue Circle invested heavily in 102 ton GLW wagons with two separate depressed centre aluminium tanks on a single bogie chassis. The tanks on these vehicles were plagued with cracks and by the 1980's they had switched back to two axle designs for new vehicles.
I think Petroleum wagons were being built in both 2-axle and bogie designs for many years. Some terminals might not have been suited to bogie wagons, and possibly the 2-axle types were cheaper to build.
The article linked above also mentions the Y25 bogie (which must be the one I remember reading about elsewhere). It's interesting that the French were researching freight bogies around the same time that BR were developing their long-wheelbase 2-axle wagons:
The French invested a lot of money in research on bogie stock in 1960's and 1970's, although the SNCF continued using four wheelers themselves. This resulted in the development of the Y25 family of bogies, enabling French wagon builders to offer large, high speed, high payload bogie stock. Fauvet-Girel and CFMF, two of the largest French wagon builders, used the Y25 bogies for their big Polybulk wagons. Storage & Transport Systems Ltd is the sole UK distributor for Fauvet-Girel and they began marketing the Polybulks in about 1974.
My understanding is that bogies are better for ride quality and protecting both wheels (well, tyres) and track from wear. Ride quality may not be a major consideration for inanimate freight, but I would have thought that reducing wear would be.
One thing to bear in mind is that a lot of general merchandise traffic in the Speedlink era open wagons and vans was not particularly heavy, so axle weights were fairly low and track wear less of an issue. Some of the 2-axle steel wagons would have conveyed things like wire-rod-in-coil or plate which would weigh far less than the slabs, blooms and coils loaded onto some of the bogie types like BAAs and BBAs.
The popularity of 2-axle aggregate hoppers is slightly more puzzling, but I suspect a lot of it came down to cost, and I'm not sure that there would have been any extra cost to the customer in BR days to take account of things like track wear from private owner wagons.
Finally, it is worth mentioning one of the few "general merchandise" type flows left on the network (although it runs as a block train). The water trains into Daventry use IZA twin vans which are a pair of 2-axle vans linked by a fixed coupling. These are not all that different from the BR-era VGA wagons, and the reason for being 2-axle is probably that the typical contents are not dense enough to require bogies to support the weight. Having a pair of 2-axle vans gives a vehicle 90 feet long which maximises volume rather than payload. Here's a very nice photo of them at the warehouse in Daventry -
https://www.flickr.com/photos/steam60163/51486008447