Because, in principle, Japanese kanji simplification only affects those in 常用漢字.
The 常用漢字 (originally named 当用漢字) was legislated to limit kanji usage within it, as the first step of gradual abolition of kanji. So no regard is given to unlisted kanjis in the first place, that is, there was no official stipulation about the rest of kanjis. People usually just followed previous customs, while some of them tried to apply it to all kanjis analogically (see 朝日字体 and 拡張新字体). The ultimate purpose of 当用漢字 was soon abandoned, and only the half-baked standard remained under the name of 常用漢字.
Today, kanji restriction is virtually no longer desired, and 表外漢字字体表 published in 2000 was seen as official discouragement against any further simplification outside 常用漢字. Since these rules only cover printed characters, you are allowed to write in simplified way by hand, in theory. But in your case, 栾 looks so different with 欒 that you might have difficulty being understood.
P.S. This article could be a good summary to this problem in English, which I haven't read through :)