Many people may prefer to go straight for the B1/B2, and there's nothing wrong with doing that in the right circumstances, but let's deal in facts here; The B Cat exams include the same Modules as the A Cat, but that's about as far as it goes.
In the common Modules, the B Cat requirement for training and exams is Level 2 instead of Level 1 for nearly all the content, and there is additional content for B Cat in some of those Modules. There are additional Modules as well. All in all it is a far harder course with much harder exams, and is not interchangeable with Cat A.
Practical training, OJT and work experience requirements for Cat B are a lot higher than Cat A, as you would expect.
For all those reasons some people prefer to start with Cat A and stay with it, or then progress over 3-5 years to Cat B while getting the experience under their belt and drawing, let's hope, some pay for that.
It's horses for courses, and not something to be dogmatic about.