Beginning of movie

The reason they went with gas in the original was due to budgetary reasons. They had always meant it to be a concentrated beam, but money and effects technology limited their options.

Godzilla's thermonuclear breath cannot be in a gaseous form because gas dissipates the farther away from the source it gets. Godzilla's breath would go all of maybe one full Godzilla body length away and then dissipate into nothing. True it would be radioactive and bake the surrounding area with radiation, but it would be incredibly underwhelming. Physics demands a concentrated beam rather than a stream of gas. Not only does gas dissipate, but it also lacks any kind of impact as it's just a slightly denser form of air in a way. Even if it's radioatice, it's just gas and that makes it incredibly limited in its capabilities.

It has to be a concentrated beam or else it doesn't work.

You realize this is Science Fiction, right? 

@Kyero

You're taking things WAY too seriously.

kyero has right about why they changed, that does seem the best reason, advances in special effects have led to godzilla with a better breath, from a vapour to a beam, to a super beam that could reach above the earths atmoshere