Why is the north side of many cites less desireable and often less
expensive,in terms of real-estate? In there some historical reason?

I don't think that the north side is especially less desirable than
any other but in almost any city there is a "wrong side of town" that
leads to undesirable living conditions and low real estate values.
Many times this has to do with "wrong side of the tracks" meaning that
there are railroad tracks or other significant barriers that keep the
less desirable areas separated from the more desirable. Why these
divisions arise is a very interesting question. I hope someone can
answer it for you.

Is it true that the north side of cities tend to be less desirable?

I could thing of two explanations, but I also have no idea if they are true.

1)  A tendency in the northern hemisphere to start settlements on the
north rather than the south shore of rivers and lakes, people
preferring to face the water and the sun.  As the setllements grow,
new residents still prefer this orientation, the better off ones
expanding east and west along the shore, while the poorer one have to
settle further from the shore, to the north.
Let all this start in the first half of the 19th century, the trains
came along, inland from the built-up downtown and better neighborhoods
along the shore, and the poor folks ended up on the "wrong side of the
tracks".

(Sounds good,  :) , but there may not a word of truth in it.)

2) Prevailing winds.  In NW Europe it has been said that the better
neighborhoods tend to be to the west of city centers because of the
prevailing
W and NW winds that blow the dirt and smells of the town to the east and south.

If the prevailing wind in many parts of America are from the south,
MAYBE this could be a reason.

Myoarin