Why are cyclones a major threat to eastern coast of india?

Cyclones, by their very definition are a major threat. There is no cyclone, SAFE for humans.

But if your question is "Why they occur in eastern coast of India", the answer is in Geography.

If you look at any world weather map, any day (on TV newschannels), you'd notice these air-climate formations progressing along the Equator (or thereabouts) in a procession going to west from east. While on the East Asian coast they veer back to Ocean (Pacific), there is no way they can do that in the Bay of Bengal, after they cross the Isthmus of Kra (Malay peninsula) from the South China Sea. Unlike the other two (Atlantic & Pacific) the Indian Ocean is blocked by Asia on its Northern side. Being hemmed in by coasts all around (Cyclones tend to go north & don't go towards Equator in the open sea, to the south) and fed by a huge amount of moisture from a warm sea, they cross the land anywhere here in India, Bangladesh & even Myanmar cycling back. The coast (obviously being a rough semi-circle) has straight stretches and at bends like (River) Krishna Delta, Mahanadi delta, Ganga-Brahmaputra Delta the sea waves 'funnel' into the land piling up to several tens of metres high. The coastal towns (what Americans call CITIES) Machilipatnam on Krishna Delta & places like Narasa`puram (close to the village where my Grandpa lived) in the adjacent Godavari Delta are chronic targets of Cyclones. Machilipatnam has a long recorded history of repeated "Gaali-Vaana" (= Wind-Rain) & "Uppena" (=tidal wave) for a couple centuries - you can look up on the 'Net, if you want.

The 1970 November Cyclone in the then East Pakistan that killed several hundred thousand laid the Geographical foundation for the civil war conditions there, government indifference & negligence, misery, destitution has paved the way for ultimate emergence of "Bangladesh" as a Nation.

Cyclones does devastating damages to homes and infrastructures.