[contravariant]

That constant is immaterial, it's just the conversion constant between two different units. It just turns out that for the units we're used the time unit is a lot larger than the space unit.

The real difference has to do with the metric on spacetime, but that gets tricky to explain. Suffice it to say that a rotation involving 2 of the spatial dimensions, and the equivalent of a rotation for time and a spatial dimension are quite different.

[trenchgun]

Is it because of th minus sign?

[contravariant]

Pretty much yeah. A metric like x^2 - t^2 is quite different from one like x^2 + y^2 (and to some extent those are the only possible in 2d)