Answer is because the 458Express is much too long for the vast majority of rifles. As such it never has a hope of becoming even remotely popular.

If you can seat long in your 458WinMag (SAAMI chambers have a long throat) then you'll cut the lead of the 458Lott which has a relatively short throat in comparison. Long throat a 458Lott and you can seat out and grab even more case capacity and really beat the 458WinMag.

The 458AccRel I use has a long-ish throat but its in a standard length action. As a result ... getting anything approaching pressure signs takes a lot of work. That long throat helping to keep pressures 'reasonable'. The fatter case allows it to equal the 458Lott.

Given your using a single shot ... re-shape the chamber with a 300RUM reamer.

The 458RUM has been done and it works. As a handloader you can control the headspace much easier and you'll pick up speed to burn ... if you want to use it. Brass is cheap too.

Cheers...
Con