Zooms became the default kit lens because they became good enough, cheap enough, and far more convenient for most buyers.

Historically, affordable zooms were bulky, slower, and optically weak, so a 50mm prime was the practical low-cost choice. Over time, lens design and manufacturing improved enough that inexpensive zooms offered acceptable image quality for everyday shooting, while giving users the flexibility to reframe instantly without changing lenses.

That convenience matches what first-time DSLR buyers usually want, especially those coming from point-and-shoot cameras where zoom range matters more than maximum sharpness or speed.

On APS-C, a 50mm lens is no longer a “normal” field of view; it behaves more like a short telephoto, so it’s less suitable as a general-purpose starter lens. A ~35mm prime would be closer to a normal view, but making shorter SLR lenses is more complex than making a 50mm because wide-angle SLR designs often require retrofocus construction to clear the mirror box. That makes them harder and more expensive to produce than a simple 50mm prime.

So the shift happened because zooms improved dramatically, buyer preferences favored versatility, and a true normal prime for crop-sensor SLRs was not as cheap or straightforward to build as the classic 50mm.