Why are the affected engine's bleed and pack switched off when there is an engine bleed under MEL?
It's not unique to the neo and the PW engine option.
Yes a single engine can supply both packs, but it's a big load on the remaining bleed system that can lead to its loss when used for a long duration.
An example from 2013 is Swiss A320 near Marseille on Sep 12th 2013, loss of cabin pressure (avherald.com). The plane was dispatched with 1 pneumatic system, the operator did not have the revised MEL procedure, and the working pneumatic system failed.
An explanation given in an Operations Engineering Bulletin for the AIR ENG 1(2) BLEED FAULT is:

(...) due to undetected failure, the remaining bleed may not succeed in sustaining this increase in air demand. In that case, it can result in an overheat of the remaining engine bleed and subsequent loss of the entire engine bleed system, leading to possible emergency descents. (...)

There is also the Airbus publication, Avoiding Dual Bleed Loss (airbus.com; PDF), from which:

Over the past years, the A320 family fleet experienced a significant number of dual air engine bleed losses. The consequences of these losses ranged from in-flight turn backs shortly after take-off, to full blown cabin depressurization events and flight diversions. (...)
The dual bleed loss events usually happen when one bleed fails, resulting in the remaining bleed on the other engine to compensate for it. (...)
In case of one engine bleed loss, the remaining bleed fails when the Fan Air Valve (FAV) does not let enough cold air reach the Precooler (PCE).


What's unique to the PW engine from my discussion with you, is that the one engine taxi procedure calls for crossbleed to AUTO, because the higher-idle of ON (used with CFM/IAE) would negate the fuel savings. For the ceo engines, one engine taxi is not a long duration, and the denser air on ground helps the precooler.
The MEL item being asked about is still as outlined by Airbus above.