It's most likely a commercial decision based upon cost and performance.

Let's take your prime at f/1.2 example.

Canon makes a number of 50mm lenses.

The EF 50mm f/1.8 II is about $100. It has 6 elements in 5 groups, 5 aperture blades and a plastic barrel. It takes a 52mm filter and weighs 130g.

Its replacement, the EF 50mm f/1.8 STM is about $125. It has the same 6 elements in 5 groups the 50/1.8 II does, but 7 aperture blades, a metal barrel, and a step motor. It takes a 49mm filter and weighs 162g. (i.e., improvements aren't always about the optics).

The EF 50mm f/1.4 USM is about $350. It has 7 elements in 6 groups, 8 aperture blades, and a metal barrel. It takes a 58mm filter and weighs 290g.

The EF 50mm f/1.2L USM is about $1500. It has 8 elements in 6 groups, and 8 aperture blades. It takes a 72mm filter and weighs 590g. And people complain about its focus shift issue.

The (now discontinued) EF 50mm f/1.0L USM had an initial MSRP of $4000. It has 11 elements in 9 groups and 8 aperture blades. It takes a 72mm filter and weighs a whopping 985g. And everyone complained about its softness.

A larger maximum aperture implies several things in the design. A larger maximum aperture means a larger aperture opening, which in turn means larger glass elements, a larger barrel, and heavier-duty mechanisms to move these larger/heavier elements. Also, the wider the lens opens up, the more you have to deal with correction for chromatic aberration and the inherently softer the lens can get at wide open--which is, of course, where people are gonna use the sucker.

With a zoom lens, this just gets more complicated, bigger, and heavier, with more possible compromises in the design and at a much higher expense. At a certain point, it's just not feasible to try and sell the lens at the increased cost, and most manufacturers have fixed on f/2.8 as that point.

Only one brand goes faster (f/2), and that's Olympus. But they're designing for a smaller-than ASP-C four-thirds sensor, which means they can probably get away with smaller elements and simpler designs because they have a smaller image circle to project.