The short answer is because, in my opinion, Ilford make the best film, paper and chemical products on the market!
Harman Photo, the parent company of the Ilford Photo black and white brands, is the only company with a full range of black and white products still committed to traditional black and white. Kodak stated in the early 1990’s it was a digital company and has been regularly deleting products since then and now has just a few films left and no papers. Fuji has never had a complete black and white range, for example they have never had a variable contrast paper and their C41-processed Neopan 400CN film was in fact Ilford XP2 in different packaging since they didn’t have the technology for their own version of such a film. Agfa went out of business several years ago and the Eastern European brands are mostly old technology with poor consistency between batches of films and papers; I need to be able to rely on my films and papers having very similar performance between batches. Oh, and I used to work for Ilford Photo for 21 years until 2002 so I have seen the products from both sides. The chemical products are what I worked on while working at Ilford; 6 years as a research scientist developing new chemical products and 15 years as the Technical Manager responsible for specifying the performance and testing of all new Ilford chemical products and ensuring the worldwide compliance to chemical regulations.