The reason is that Job was not dictated to Moses by Hashem for the purpose of being put into the Torah. The words of the Torah were specifically for the history, halachos, and hashkafa of Bnei Yisrael. Thus Moshe wrote it at the lower level of nevua set up for Kesuvim.

The Chumash is like the Neviim in that they were given as a message by Hashem to the Navi with instructions to repeat them to Bnei Yisrael. They were then written down (also as instructed by Hashem. That is why we see Vayedaber Hashem el Moshe Laimor constantly. Just as Moshe was given the prophecy, he was explicitly commanded to tell everyone. This is from a shiur that I attended on the Rambam. and part of a discussion on the concept of "navi sheker"

If someone repeats the nevua without being commanded, he is a "false prophet".

The talmud in Bava Basrah 14b gives who wrote the actual books that the Anshei Kneses Hag'dolah compiled into the permanent books of the Navi that we have. 

Something written on his own or for his own purpose is in Kesuvim as I state in I point to the meforshim in Yirmiyahu that discuss the matter as part of the discussion of Hananiah ben Azur.

Since Hashem did not dictate Iyov to Moshe (nor the Psalms that Moshe wrote) and command him to put them in the Torah, then they are not found in the Torah. I realize that this is somewhat circular reasoning, but in this case we have to say that not finding them in the Torah is the evidence that they were not commanded to be there, since only those things that were explicitly commanded (and dictated) are to be found in the Torah.

Similarly, only those things that are told by Hashem to a navi (such as Shmuel, Nasan, Gad, Yechezkel, Yirmiyahu, etc) and commanded to be passed on to Bnei Yisrael (and written) as nevua can be part of Neviim. Anything else, even if written with Ruach Hakodesh, can only rise to the level of Kesuvim. Note that Kesuvim are only those things that Anshei Knesses Hagedolah accepted as part of Kesuvim. There were other writing that were not accepted and nothing written after Anshei Knesses Hagedolah (that is after Megillas Esther) closed the canon can be accepted as such. The 120 members of Anshei Knesses Hagedolah included the last of the neviim. That is why the books of the Maccabees are not part of Kesuvim. For example, David Hamelech compiled all the songs of praise that had been sung since Adam and put them into a single document (Tehillim), or the wisdom of Solomon was compiled and put into a volume for future study (Mishlei) - and accepted by Anshei Knesses Hagedolah as part of Kesuvim.