MOTM

The primary reason is simply mechanical.  Solid is not very flexible.
Stranded can be quite flexible.  But, for wiring where it is to be put into
place and not moved around, there is nothing wrong with solid.  The "skin
effect" (yes that is what it is called) is only an issue at very high
frequencies.  There is a difference in the ampacity of the same size
stranded and solid wires.  But, that is not an issue for synth power supply
uses.  That only applies to us high voltage guys who rate the loadability of
wire by the conductor temperature, how much it sags between the poles, and
how much the wind is blowing.

So, solid is OK, if it is not to be moved around.  Most house wiring is
solid.  However, I would use some nice stranded stuff.  I pick up lots of
nice utility grade stranded wire scraps from constuction sites at work.
Pieces under 10 feet are virtually usless to us.  However, they are usually
just right for synth work.  I will ba happy to send nice 12 or 14 guage
stranded wire for synth power supply wiring inside their cabinets to anyone
that asks and tells me how much they need.

Larry Hendry