Because 鹼/鹸 isn’t a 常用漢字 and the 新字体 isn’t otherwise established as the normal form, so it really depends on what the individual wants to use, or more likely they don’t care and just use whatever pops up first when converting せっけん
旧字体 vs 新字体 is an absolute mess. There are some non-常用 characters that are frequently 新, like 掴、蝿, or 涜 and some 常用 ones that are actually 旧, like 彙、剝、茨、恣、淫、葛、僅、惧、煎、嘲、詮、喩、箋、捗、溺、塡、頰、籠、餌 (and a few others). For some of those the version that’s most often used is 新字体, even though it’s not official as per the 常用 list
The reason for this is mostly that the Japanese government had a change of heart on whether or not to rigorously simplify between the original 1981 常用漢字表 and the update in 2010, so much so that in 2004 (iirc) the main JIS kanji standard changed a lot of characters that were previously simplified back to their traditional variants
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Because 鹼/鹸 isn’t a 常用漢字 and the 新字体 isn’t otherwise established as the normal form, so it really depends on what the individual wants to use, or more likely they don’t care and just use whatever pops up first when converting せっけん
旧字体 vs 新字体 is an absolute mess. There are some non-常用 characters that are frequently 新, like 掴、蝿, or 涜 and some 常用 ones that are actually 旧, like 彙、剝、茨、恣、淫、葛、僅、惧、煎、嘲、詮、喩、箋、捗、溺、塡、頰、籠、餌 (and a few others). For some of those the version that’s most often used is 新字体, even though it’s not official as per the 常用 list
The reason for this is mostly that the Japanese government had a change of heart on whether or not to rigorously simplify between the original 1981 常用漢字表 and the update in 2010, so much so that in 2004 (iirc) the main JIS kanji standard changed a lot of characters that were previously simplified back to their traditional variants
If you want to know more, there’s a series of articles written by an employee of the 朝日新聞 where he painstakingly goes over every character added to the 2010 常用 list that either is or isn’t 新字体: http://www.asahi.com/special/kotoba/archive2015/author/index2.html%3Fpage=5&id=0043.html
They are from chinese fonts, not kyuujitai.