In the New52, Superman has Kryptonian Armor for his costume instead of something resembling tights.

Ultraman comes from an alternate earth, but an earth where Kryptonians should have had the same technology as they did in the universe featuring New Earth.

Why then, is his costume something that resembles tights, instead of the Kryptonian armor New Earth Clark wears?

The technology may be the same but their mindset is different. From what little we seen the kryptonians of Earth 3 may be into making more weapons than armor. Ultramans's cape is not invulnerable as the ends are tattered.

why does he get stronger around kryptonite and weaker around yellow sunlight?

answer:

Even though its called armor, Superman’s costume actually more like ceremonial uniform or something like that; he said it himself in Annual – “my costume doesn’t make me any more invulnerable or stronger than I am already”. So far his cape was shown to be the most armored part of his uniform ;)

Events not necessary played out similarly in two worlds. Superman got his costume on Collector’s ship – while Ultraman… Well, we don’t know where he got his costume.

So that’s probably question of design and origin rather than any noticeable difference in technology between Kryptonians on those two Earths.

@sanohibiki:

Agreed, nice answer. I forgot that his armor was not actually armor, since it looks so much like armor.

@ssj4jw:That could be...why focus on retracting clothing when you could focus on making something to hurt someone...

Because the artist made it look like that. They told him, draw evil superman and he did not bother checking the origin of supermans costume in-universe.

If you just switch the U for an S then you have the perfect superman costume

Agreed. Maybe make the belt buckle yellow instead of grey also.

@muyjingo: agreed

@muyjingo:

That's you making quite the assumption.

Just because they're both kryptonians and were born on Krypton, that doesn't makes them equals. Look at their powers for the first example. What kills one fuels the other. Has a second example, look at Ultraman origin story. His parents weren't loving, caring good people. Krypton in general looked like a warzone.

And has we've seened through the multiverse, not all Krypton's developed the same way.

The same question you could also make about Earth-2. Both Superman and Supergirl of Earth-2 had suits that looked nothing like armor, and also they didn't have the shape-shifting options that the one's of Earth-0 have.

Doesn't matter.