why do we use 'non-increasing' instead of decreasing?

Personally I find this among the most awful terminology in existence. It starts with the ambiguity present in "increasing" and "decreasing" themselves: common sense would have that this means getting ever larger/smaller; yet (if I take Wikipedia as reference) both the terms monotonically increasing function and monotonically increasing sequence allow for (local) constancy. (It seems unlikely that the purpose of "monotonically" is to weaken the notion following it; rather it seems to indicate that a formally defined rather than colloquial notion is meant.) So if there is doubt about what a bare "increasing" meant, the proper remedy would be to always accompany it with a disambiguating "weakly" or "strictly"; this would settle the matter.

For some reason however many people seem to find that "nondecreasing" is preferable to "weakly increasing". I work a lot with integers partitions, which most authors introduce as nonincreasing sequences of integers (with finite sum). Clearly what is meant here is not the absence of "monotonic increase" between successive integers, since that would imply strict decrease. One might conclude that when using negative terminology, people implicitly revert to the colloquial rather than formal meaning of the base notion. For comparison, even here in France, where "négatif" is taken to include $0$ (as does "positif"), few people would be willing to interpret "entier non-négatif" as designating integers${}>0$.

However, even apart from the fact that negation does nothing to remove ambiguity from a notion, there are other drawbacks specific to this case:

• Nonincreasing is not the negation of (strictly) increasing for sequences of length${}>2$, and should therefore be carefully distinguished from "not increasing". The sequence $0,1,-1,2,-2,3,-3,\ldots$ is all of "not increasing", "not decreasing" and "not constant"; however, it is neither of "nonincreasing" nor "nondecreasing", but it is "nonconstant". A nice mess.
• In the presence of partial ordering, having "nonincreasing" mean "weakly decreasing" is even less justified; here weak decrease is stronger than the absence of strict increase even for sequences of length $2$. I think what is needed in such context is almost never "nonincreasing", even between successive elements. For instance a "plane partition" could be defined as a weakly decreasing sequence of partitions (for the containment-of-diagrams partial ordering); saying "nonincreasing" here would be utterly confusing.

If one must absolutely use negative terminology, then it would have been much better to use "nowhere increasing" rather than "nonincreasing" (and even then only for total orderings).

In conclusion: if you want to be precise, it is better to say what you mean rather than to say what you don't mean (or even to not say what you are nonmeaning).

There are two possibilities:
* increasing and strictly increasing
* nondecreasing and increasing
Someone who switched the terminology from one to the other perhaps did it thinking that the more common notion should have the shorter name. But not everyone has switched terminology, so now we have the two systems existing side-by-side, which is, indeed, confusing.

Similar situations:
* nonnegative and positive
* positive and strictly positive

also
* $A \subseteq B$ and $A \subset B$
* $A \subset B$ and $A \subsetneqq B$

It might be easier to say something helpful if you explain why you think it doesn't make sense. It makes perfect sense to me -- "non-increasing" means it doesn't increase, i.e. doesn't become greater, i.e. stays equal or becomes less; "decreasing" means it becomes less -- those are the standard meanings of the words in everyday language, and it seems it's the other convention, the one that uses "decreasing" for "non-strictly decreasing" and "strictly decreasing" otherwise, that's in need of justification because it departs from the everyday usage of the words.

About your edited question: It sounds as if you're assuming that there's one standard convention and people who don't follow it are confused and making mistakes. In my experience there are two different conventions in use, one where "increasing" means "non-strictly increasing" and one where it means "strictly increasing". It seems to me that it's just a matter of taste whether you'd rather have shorter words ("non-strictly increasing" being a bit verbose) or whether you want to stay close to the everday usage of the words ("increasing" in everyday usage meaning "strictly increasing").