Probably because of size. Some DIP packages got 1 VSS and 1 VDD, but SOIC package got 2 both.
Iv seen in a data sheet ... that 1 pin sink current max is 25 mA but whole pic pins got sum of 200 mA. This logic sequence continues: Cant sink 200 mA from only 1 small VDD pin (SOIC package).
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My questions for PROs
How to power pic internal RTCC. If main power source is AC adapter.
How to connect a battery and how to use it when it is only needed.