Whenever I look at most American probe start metal halide ballasts, I mostly see them as CWA ballasts. In addition, there were some American 277v probe start metal halide chokes that did exist. I am just wondering if there were any HX probe start metal halide ballasts that did exist for the US market? Just curious about that because I understand that HX ballasts seem to be more versatile in terms of lamp choices.

The MH need higher OCV for the same arc voltage, which makes the inductor (less likely because it would need mains above 300V) or the HX pretty much the same really independent current source as CWA when viewed from the load side, but way more sensitive for any mains voltage variation.
Because the HX needs a PFC capacitor in most installations anyway, the HX has no cost benefits over CWA anymore. On the contrary, because the HX is a lag ballast, both primary and secondary have to be designed to handle the high VAs the high OCV implies. The CWA is a lead ballast, so the core does the PF correction, so the primary then has to handle only the real power, so could use thinner wire, so at the end allow the CWA to be cheaper than the PF compensated HX. Hence no intention to go for HX.

You should not forget unlike Europe, US market did accept different ballast for each lamp type, even when speaking about the same wattage.
European market accepted maximum two choke types for the same wattage, yet they have to cover all high pressure lamps of the given wattage in use.
The strong motivator for the US market was, the ballasts have to be step up transformers (with the related cost, size, weight and losses penalty) anyway, so it makes sense (when making the standards) to optimize the lamp arc voltages for best lamp performance and then design the ballast with the OCV each lamp needs.
In Europe (in fact mean all 230V countries) having the OCV anything else than the mains would mean the ballast becomes heavier, lossier and more expensive transformer instead of the simple chokes, so lamp parameters were optimized to get the best from what the compatibility with a 220V OCV ballast allows. And that lead to just two types: One for lamps with stable arc voltage and stable arcs, so allowed to go to full "mains/2" limit or even a bit beyond (for the rwally high power lamps), so leading to the 100..130V arc voltage range. And the second for lamps exhibitting either less stable arcs or large arc voltage variation, leading to 70..85V nominal arc lamps.

I do understand that using ignitors is often necessary in order to compensate for low ballast OCV.

Europeans uses series chokes, so they need low voltage ignitor to start probe-start MH lamps.
As the Americans uses autotransformer ballasts, they can simply rise the OCV of their ballasts.
Some members here, said that the sodium-scandium chemistry of American MH lamps, performs better on CWA ballasts than on reactor ballasts, as CWA ballasts provides constant voltage and that this chemistry is very sensitive to mains voltage variations.

I did find an Iwasaki catalog that says that their CWA ballast compatible metal halide lamps can operate just fine on MV ballasts using low voltage ignitors as well.