Tom Riddle's mother - an important question...

[Cindy]

No: HPFGUIDX 68853

Hello All,
I'll just get straight to the question, and if anyone has any theories
or answers about this, I'll be glad to hear them!
Tom Riddle is the heir of Slytherin because his mother was a direct
descendant of Salazar Slytherin - so the original Slytherin's blood
was running through her veins, and is now in Tom Riddle...
That means that Tom Ridddle's mother would have been in Slytherin
house at her time in Hogwarts, assuming that she went to Hogwarts.
Where did Salazar migrate to when he left Hogwarts castle? Somewhere
near Durmstrang, perhaps - I don't know... Anyway, the question is:
Why did Tom Riddle's mother marry a muggle?! She must have known
something about her family tree and bloodline (as all purebloods are
interrelated and seem to take great pride in knowing where they come
from - tapestry in ootp).
It doesn't really make sense.
-Cindy (Snape's girl)