Its always the nuke cliche

I agree that it is a bit of a cliche, but I think it makes some sense. I'm just theorising here, but perhaps standard bombs are not destructive enough to kill some contagions. Nuclear bombs actually break apart the basic structure of matter (as I understand it), whereas standard bombs may not be able to break up the cells of a certain contagion. Also, I think that it would be unlikely that something would survive a direct nuclear hit, let alone be able to grow from it, so I think they were quite right in wanting to nuke it.

Perhaps nuke is the easiest way to irradiate a 30 miles area?

Which of non nuclear bomb can irradiate a single place for 100 years?

Most convential explosives do not reach a high enough temperature to destroy various biological and chemical elements. They just release a concussive explosion that does damage to structures, but don't "burn" enough to eradicate a disease. Napalm might work on some agents, but a cluster bomb, or others like that, are designed to destroy buildings/structures/equipment/etc. Viruses and other biological organisms would certainly survive.
However, I do agree that always relying on nukes to end the problem is getting a little old; however, it is really the most realistic and militaristic/logical response to a biological disaster on the level that Andromeda wrought.
And they did eventually discover that the nukes would have an adverse effect and attempted to abort. Of course, that didn't really work as the strain was "smart" hahaha.

The thermonuclear device was not a cliché in 1969, when Michael Crichton wrote The Andromeda Strain. As for why they used it, from the opening of Chapter 8:

Directive 7-12 was a part of the final Wildfire Protocol for action in the event of a biologic emergency. It called for the placement of a limited thermonuclear weapon at the site of exposure of terrestrial life to exogenous organisms. The code for the directive was Cautery, since the function of the bomb was to cauterize the infection—to burn it out, and thus prevent its spread.

Though this movie did vary from the book, it was supposed to be a nuke if they wanted to have anything to do with the book at all. I still claim Michael Crichton probably rolled over in his grave after this movie.

"It's better to be hated for who you are than be loved for who you aren't."