Why atherosclerosis taken place in artery not in vein?

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Well, because "athero" means artery and "sclerosis" means...

Oh, okay. The reason atherosclerotic disease is more a problem of arteries than veins is mostly to do with the nature of the walls of each. Arteries have three layers, as do all blood vessels, an intima (innermost layer), an intima media (in the middle) and tunica externa or adventitia (on the outside). In arteries these layers are much thicker and are invested with more muscle than in arteries. This is probably why, when fatty streaks (collections of cholesterol) form, macrophages (a type of white blood cell) are also around and react with them, forming atherometous plaques in the wall. These plaques can then rupture and heal over and over again. Eventually, a rupture can either occlude an entire vessel, causing the death of whatever that vessel feeds, or the rupture-and-healing process can cause so much calcification and scarring that the vessel becomes "stenotic" and can't distend enough to carry the blood it needs to carry. If it blocks up entirely, you have an infarction. If it just isn't carrying enough, you get angina (if it's in the chest) or claudication (if it's elsewhere).

Veins are much, much thinner and more distensible than the muscular arteries and the same process doesn't occur in their vessel walls. However, damage to the inner wall of veins can cause a blood clot to form. If it gets big enough, we call it a "thrombosis" and it too can cause the same kinds of problems--it can block up the whole vein, leading to venous insufficiency and swelling in the area distal to that vein, or if it breaks up, it can form an embolism, travel through the body and cause major problems, pulmonary embolism being the most well-known.

WRONG,
Although atherosclerosis means hardening of the arteries (by deposition of cholesterol and calcium cholesterolate), it also accumulates in the walls (endothelial cells) of the veins and capillaries......
We do use the term atherosclerosis, as a shortening to define the total pathology involved mainly, the "BAD" cholesterol, which is low density one......(LDL) carried by lipoproteins....

Check your definition of atherosclerosis in your textbook,,,,You will find interesting to know, that cholesteroil can accummulate in lyphatic vessels, veins capillaries, etc.....