It's an availability thing and the pricing for boron rods forced change. Manufacturers moved to as ex. sapphire and other gemstones for cost primarily but availability is also a major issue. This is a quote right on Ortofon's site acknowledging the change a while back.
They do acknowledge Boron is a better choice , something I always felt showed in the sound characteristics.
"Over the decades, there has been an increase in the use of Boron. Elemental Boron is mostly used in high tech applications. One of Boron's benefits for our modern technology is its use in neodymium magnets, which are an alloy of neodymium, iron and boron. Boron has also been applied in manufacturing of phono cartridges: Boron cantilevers are strong, stiff, light and 100% inert. From all possible options Boron cantilever is the best, but unfortunately extremely difficult to obtain."
They do acknowledge Boron is a better choice , something I always felt showed in the sound characteristics.