The Whole Life is a Tradeoff
As always in a real life situation decisions must be made based on contradicting requirements. In this case optimal power supply and easy PCB design are an examples where both are good considerations but hinder each other.
Consideration #1 PCB layout
For simple PCB design it's quite handy if supply power and ground are on opposite 'ends' of a chip (*1). Having one at the 'upper' end (where the mark is) and the other at the lower end (*2) allows a design with a power bar running along one side and a ground one along the other. With a little luck when routing and the usage of rails (*3) it may allow the use of single sided boards with a minimum of discrete wiring (jumpers) - or at least keep a design within two layers. Multiple rows of chips lead to a design with a pair power traces (or rails) running along the middle as seen on many boards.
Consideration #2 Power Supply Quality (Buffering Capacitor)
On the other hand, for a good power supply it's handy to have a right-sized capacitor as close as possible (short routing/wires) to the power supply pins to buffer switching needs. The higher switching frequencies get, the more important this is. Placing power and ground at two adjacent pins would allow adding a capacitor right next to them.
Also, placing both of this pair near the middle of an edge further shortens the wire length, as the internal part of these pins (which in turn are bonded to the die) are here the shortest of all.
Of course routing of power lines will now be rather complex among all the signals. But still manageable - especially when again using rails (*4).
Ok, But Then Why Opposite?
It is desirable to have the same voltage between ground and power across the whole chip area (or as much as possible at least). Unlike simplified assumptions, semiconductors have a rather high resistance, about 10 to 1000 times of copper. Increased by the fact that conductanc is not only defined by the material, but also conducting diameter, power distribution across a chip becomes a serious issue (*5). And no, impedance isn't of much relevance for power supply, as we do not have an alternating voltage (6).
If power was supplied by adjacent pins, the region where these pins are bonded in would enjoy full voltage, while the areas at the other 'end' would get a quite lower supply resulting at least in lower maximum performance if not worse. By placing them on opposite sides of the die, the voltage loss is somewhat compensated over most areas. None gets maximum voltage, but all get about the same (*7) which again is something easing circuit design of the chip.
Putting both on opposite positions (horizontal (*8) or diagonal) is therefore mandatory.
It's important to note that pins using a 'diagonal' layout (like TTL or most other) also end up in the middle of opposite edges of the die - after all, it's square (mostly) and pins are bonded all around.
Bottom Line: Zilog's solution scales better with higher frequency and allows better signal quality, but requires a little more complex PCBs.
*1 - At least for single voltage ones that is.
*2 - I doesn't matter if both are on the same side (left or right).
*3 - These are metal bands with pins in regular intervals like .6 inch or alike. With rails, the routing of power can be eliminated for major parts of a PCB. This was even more important with early automatic routing systems removing a whole dimension of complexity.
*4 - Now standard .6" or 1" rails may not work as well, but it's still better than routing them among all the signals.
*5 - That's also the reason why these huge modern chips have not just one or a few pairs of power pins but sometimes hundreds.
*6 - It is of course relevant if we look at digital lines.
*7 - Yes, there is still variation due to routing and so on, but it's much less than without this consideration.
*8 - The PIO breaks this a bit by using 11/26 instead of 11/29 in favour of keeping all Port B pins adjacent. A little variation doesn't kill the principle.