Because he was not as wrong as we think he was. The sounds of war that Yehoshua recognized were the sounds he had heard in the fight against Amalek. At that time, Amalek had attacked the stragglers, those who for whatever reason were left outside the protection of the ananei hakavod. Now, said Yehoshua, “Kol milchama bamachaneh,” that sound of battle with Amalek were in the very midst of the camp. What was the tumult over the eigel if not a resurgence of the evil of Amalek? (Sefas Emes)
Moshe davened for Bnei Yisrael, Hashem’s anger was assuaged, the 3000 who worshipped the eigel were killed, but all was still not right, as evidenced from the fact that Moshe moved his tent outside the camp. It would take forty more days of work for Moshe to restore unity in Klal Yisrael and undo the damage caused by the eigel. The Torah then again comes back to Yehoshua: “…V’Yehoshua bin Nun na’ar, lo yamish m’toch ha’ohel.” (33:11) Why does the Torah shift gears and give us half a pasuk about what Yehoshua was doing,right in the middle of the description of Moshe's "cleanup" efforts?
The Netziv writes that Moshe had a tremendous amount of work to do in rehabilitating the damage done by the cheit ha’eigel. Even though Yehoshua is described here as a “na’ar,” in reality he was in his 50’s and had been learning in “kollel” at the feet of Moshe for years. Wouldn’t Yehoshua have been the natural choice to help Moshe in his efforts to repair the damage that had been done?
That’s exactly the thinking that the Torah here tells us is wrong. Moshe’s job was being the manhig ha’dor, being the leader, and with that came the responsibility of shouldering the burdens and work that was needed. Yehoshua’s job was being a talmid engaged in learning. Doing that job means no matter what the crisis, no matter how important the needs out there may seem, “lo yamish m’toch ha’ohel,” one must keep learning.