It is because modern zoom eyepieces are just as good as were the Wide eyepieces earlier, and offer the flexibility of a zoom. Due to this, new customers have overwhelmingly preferred the zooms over the fixed primes, and not only Leica but also Swaro and Zeiss have pretty much phased out their primes as a consequence. The only premium maker to still offer a wide range of primes is Nikon, and this is due to the regrettable fact that their zooms have not yet caught up.
Now that there is the dedicated extender for the Leica 25-50x zoom, really the only thing you'll miss over the hypothetical alternative of a range of four or five primes is a minimum magnification of 20x.
Pretty much starting from the Swarovski 20-60x zoom (certainly the second generation version), Swaro's primes started selling slowly, and after the 25-50x Wide zooms from Leica & Swaro the primes really became redundant.
The best zooms now have the field of view, eye relief and edge sharpness of the best primes, and give up nothing in centerfield sharpness, brightness and contrast.
Kimmo