Particles

Hello,

I just stepped in this doubt and got really interested. Why do the は、へ and を characters sound different when written as particles? I mean, at the beginning I thought the pronunciation simply had changed along the time, but if so, why weren't the characters replaced by the ones that correspond to the actual pronunciation (わ、え and お）?

For example, in portuguese, which is my mother tongue, we used to say "cousa" (it means "thing"). Latter it changed its pronunciation to "coisa", but so did the writing. Why didn't it happen with some of the Japanese particles?

Japanese script reform - Wikipedia, the free encyclopedia
Modern kana usage - Wikipedia, the free encyclopedia

Unlike other historical kana usages of は, へ and を than particles, which also pronounced as wa, e and o respectively, these three particles maintain their historical kana form to show these are particles.
e.g.
historical
「かはづ」はカジカガエルのことであらう。
modern
「かわず」はカジカガエルのことであろう。
(Notice that は changes to わ in a word かはづ/かわず, unlike the particle は.)