It seems it’s due to the synthetic not being fully recognized for equivalency in the US pharmacopia per this article: https://www.npr.org/2023/09/23/1200620535

It seems it’s due to the synthetic not being fully recognized for equivalency in the US pharmacopia per this article: https://www.npr.org/2023/09/23/1200620535/vaccines-are-still-tested-with-horseshoe-crab-blood-the-industry-is-finally-chan#:~:text=The%20blue%20blood%20of%20the,LAL%2C%20was%20invented%20decades%20ago https://www.npr.org/2023/09/23/1200620535/vaccines-are-still.... “ The blue blood of the horseshoe crab clots when it comes into contact with bacterial toxins, which helps technicians identify contaminated products. A synthetic alternative to the blood-derived testing ingredient, called limulus amoebocyte lysate, or LAL, was invented decades ago. Alternatives have since become mainstream; most of the east coast bleeding companies now also sell tests made with a synthetic, not just LAL, and the European Pharmacopoeia considered the synthetic ingredient equivalent to the crab-derived one in 2020. But since scientists at the U. S. Pharmacopeia had not yet done the same, drug companies that wanted to use them faced extra regulatory hurdles in the U.S.”