Methinks it's physics

The PH134, which can reach 900F is 2800W and the Breville only reaches 750F with its 1800W. That is a pretty strong indication that to go above 750F you need more than 1800W
You need more than 1800W to get to 900F in a reasonable time, and be able to bring the temperature back after you open the door and add the pizza. It might be possible to add a lot of insulation, but it would make the oven much bigger and it's still unlikely to be able to bounce back to 950F once you open the door to insert the pizza
Given that it's hard to get more than 1800W in a domestic USA/Canada kitchen, any oven above 1800W/15A will have a small market. In theory you could get a 120V/20A circuit, but at that point having a 240V feed is more common. And the small market that is willing to use 240V can easily get a PH134 (easily, not cheaply

), making it less appealing for a company to spend the money to design and market such an oven
The same limitation caused problems to the espresso machine market years ago: most of the higher end espresso machines needed more than 1800W (especially the dual boiler ones), and it took time for the market to develop: true espresso aficionados were using 240V or having a 20A plug installed, until demand grew to the point where dual boiler machines were developed with enough circuitry to never draw more than 1800W at once. That is easy to do for an espresso machine, which needs much, much less than 1800W when at a stable temperature, but would be really hard to do, if not impossible, for an oven