To be fair, on this particular point it can be incompetence. Telecoms have absolutely no skilled engineering capacity (third-world body shop is as good as it gets) and the entire thing runs on decades of duct tape and outdated, unsupported and vulnerable software.
The most likely reason for this not to be possible on prepay is that prepay and postpay are managed by completely different systems and making the prepay system work with eSIMs would be too difficult, or maybe they tried, it broke, they rolled it back and have a "TODO" to fix that (of course the TODO will never be addressed).
