Why calf thymus DNA is widely used instead any other body part?
I have crawled through google and many, many journal articles. What I can make of it, is: 

"It is generally agreed that from mammalian cells (as, for example, calf thymus..." are capable of yielding high molecular weight DNA from cells, with very little protein present to decrease purity. [From Welsh and Vyska]

Additionally:
"Calf thymus gland is a fairly good source of replicatiue deoxynucleotidyl
transferase (deoxyribonucleic acid polymerase)" [From Yoneda and Bollum]



So we have a good DNA coming from the calf thymus. Calf thymus is also something we have in great abundance (worldwide slaughter of cattle) and I don't know of any other demand for it (don't know of anyone eating it?). So it should be effective and not too expensive, relatively.

Perhaps there is a difference between the thymuses (thymi?) of calves and adults, given the immune role of the thymus, it's possible. But I can only speculate on that.



And yes. It does seem a bit weird. But it works and everybody does it, so we seem stuck with it.



My two sources:

Richard S. WELSH / Karel VYSKA, Relationship between the Purity and Molecular Weight of Calf Thymus DNA, Hoppe-Seyler's Z. Physiol. Chem.
Bd. 362, S. 969-981, Juli 1981

M. YONEDA AND F. J. BOLLUM, Deoxynucleotide-polymerizing Enzymes of Calf Thymus Gland, THE JOURNAL OF BIOLOGICAL CHEMISTRY, Vol. 240, No. 8, August 1965