As @ZhenLin said in the comments, there are three particles left unchanged in the reform of the usage of kana: は へ を.
They are left unchanged because they are so widely used, and changing them would result in too much in the writing form. I quote from a book*(I don't know the book, so the contents are in fact from wiki):
*現代かなづかいの精神・抜粋(国語シリーズ8/文部省著、統計出版・昭和27年3月)
The spirit of the modern Kana usage (an extract) (Japanese Language Series 8, By the Ministry of Education, Culture, Sports, Science and Technology)
「本を読む」のををば、をと書く例外をことわっている。これが問題である。なぜこんな例外を許したか。例外にせずに、これも「お」と書いたらよいではないか、という非難がごうごうと聞える。これには、こういう理由がある。
もちろん例外は、よくよくでないかぎりは設けないほうがよい。委員会でもそれは皆心得ていたことなのである。だから、この例外を設けたのは、よくよくのことなのである。
およそ改革は、ことに万人の所有である言語の改革は、まさつの少ない、万人のすぐついてこられるものでなければ、案がいかにりっぱでも、机上の理想論に終って、実現ができない。理想としては、だれもだれも助詞のををもおにしてしまいたい。しかし、助詞というもの、ことに「が」「の」「に」「を」「へ」「は」などは、最もたくさん出てくる。〔中略〕いちいち
これわ それわ わたくしわ
それお これお わたくしお
これえ それえ わたくしえ
というように書くようになると、あまりにも、今までと変りすぎて異様さが目だち、ちょっと実行の手がにぶる。この助詞さえ、もし今までどおりにして置いてよかったら、他の点は、漢字で書くとほとんど隠れて、新かなづかいも、大部分今までどおりで済む〔中略〕助詞だけは漢字で書けず、いつもかなであって、必ずひっかかる、いちいち直すにかかる手もうるさいが、見る目にも抵抗が多過ぎて、すぐ実行できるか、あやぶまれる。これが、大新聞社側の決定的な意見であった〔中略〕いかにも、「わ」「お」「え」が、目にたって、一見異様であって親しめなかった記憶が、ある委員たちにもあったのである。
大事の前の小事である。実行できない案では、いかに美しくってもなんにもならない。要は実行できる案でなければ、一時強行されても、少しでも無理があると、動天返しになる憂いがある。
そこで委員会も、助詞を元どおりにのこすという妥協案を決定するよりほかにしかたがなかったようである。
"There is exceptions that を in 本を読む would be written as を. This is a problem. Why the exceptions are permitted? Why not write this as お and elimination all exceptions? Often there are criticisms about this. There is the following reason for this. Of course exceptions should not be allowed unless it can be avoided. The committee is well aware of that. However, these exceptions are the ones that cannot be avoided.
"Reform, especially the reform of language that is used be everybody, unless it can be followed by everybody in a short time, it will only be an ideal plan on the desk, and cannot become true. As an ideal (plan), everybody wanted change the particle を to お. However, the particle, especially 「が」「の」「に」「を」「へ」「は」 are most widely used. If we change them all, and write as
これわ それわ わたくしわ
それお これお わたくしお
これえ それえ わたくしえ
there would be too much change from the original writing, and would be hard to be put into practice. If only these particles are written as the original, other changes will mostly be hidden when kanji is used. The new kana usage will not have too much change in actual writing. (...) Only the particles won't be written in kanji, and are always written in kana, so will certainly be affected (by the reform). Changing all of them will cause too much effort, and it will cause too much resistance from people who read it. This is the decisive comment from the big publishing companies. (...) some members of the committee also (think) that 「わ」「お」「え」will stand out in the text, and make the text not look like normal Japanese.
"This is a 'little thing' before a big reform. A plan that cannot be put into practice will be nothing however good it is. If it is not a practical plan, even it is forced into use, it maybe reversed as long as there are something (unreasonable).
"Thus, the committee has to use the plan that keeps all the particles."
You should see that を is only kept as a particle, its appearances in other words are all changed to お, just like ゐ and ゑ changed to い and え.
を
other than as a case particle is practially ignorable. Whileを
helps to identify an accusative case particle,は
andへ
actually make it difficult to locate these particles. For this reason, in Japanese braille,を
is written asを
, but the particlesは
andへ
are written asわ
andえ
respectively. – Dec 28 '11 at 04:19は
andへ
, which have two pronunciations, but forを
, there is only one pronunciation 'o', and is unambiguous. Therefore, that argument makes no difference forを
. The change from "wo" to "o" is due to a genaral phonological rule, just like "wi" to "i", "wu" to "u", and "we" to "e", and has nothing to do with one-to-one mapping, unlike withは
andへ
. – Dec 28 '11 at 05:46買います
or買えません
instead of買ゐます
or買ゑません
while we write本を
instead of本お
? Your argument is not answering this, which means it is irrelevant to the question. There was an idea of getting rid of the ambiguity of theは
andへ
. ハ行転呼 does not apply unconditionally.を
didn't have such problems. – Dec 28 '11 at 06:08買わない
, which means that the stem iskaw-
. Since the epenthetic vowel inserted before the polite-mas-
isi
, it would be logically expected that the underlying form is written askawimasu
or買ゐます
. And since the potential affix is-e-
, it is expected that the underlying form iskawemasen
or買ゑません
in present Japanese. The form買ひます
is reflecting the verb stem before the historical ハ行転呼 change, and is not relevant for present Japanese. – Dec 28 '11 at 08:34を
. I never said it should have been writtenお
. – Dec 28 '11 at 09:07