I know that there have been endless discussions about this. Here's the response that I got from Penhaligon's about their reasons for reformulation:
Regarding the reformulated shaving soaps, we have decided to stop using tallow for ethical reasons. We are trying to limit the use of animal-derived ingredients in our products. We recognize that this move is unfortunate for customers who are loyal users of the tallow based soaps; however, we feel that the new vegetal based formula is equal, if not superior in quality compared to the original. Feel free to let me know if you have any further inquiries.