Can anyone answer the questions above? Because this really confuses me.
Any help is appreciated.
Thanks in advance,
Bajiru
Dakuten change the consonant of a kana in a specific way - namely, they add voicing to a voiceless consonant. So さ /sa/ becomes ざ /za/, and so on. But what about sounds that are already voiced, such as /n/? What in the world would な゛ even mean? You can't make /n/ any more voiced than it already is.
As for handakuten, they exist specifically to add /p/ back into Japanese. When the kana were originally created, what's now the /h/-row represented *p (or rather *ɸ, descended from *p in Old Japanese). This *p changed to modern /h/, but Japanese has since regained /p/ from Chinese loanwords. Thus, they needed some way to write this new /p/, so they modified the old *p kana with a handakuten to make them the new /p/ kana. Since this is an unsystematic change - it doesn't affect a class of sounds in any particular way like the dakuten does - it's not clear what a handakuten added to a different kana would even mean.
So in short, the reason that dakuten and handakuten are restricted to a subset of kana is that if you added them to any other kana, there would be no clear interpretation of the resulting combination.
There are some odd edge cases. Dakuten can be added to vowel kana in manga and similar environments to indicate something like loudness or intensity (e.g. あ゛あ゛あ゛あ゛あ゛!); though this is a bit uncommon. ゔ has been created as a way to transcribe [v], though it's not consistently used (people often just write ぶ). And k-row kana with a handakuten are sometimes used in linguistic works to specify pronunciation with [ŋ] as opposed to [g] (e.g. か゜is specifically [ŋa], not [ga], when in normal writing が could be either). Those are the only cases of unusual combinations, though.
Why do not all かな get dakuten? And why only the は ひ ふ へ ほ へ kana get handakuten?
There are sounds first, letters are made to denote them.
[半濁点]{handakuten} was devised to denote [破裂音]{haretsuon} or stop consonant.
If you have enough ways to denote the sounds of your language, you need not have any more.
Stop consonant
In phonetics, a stop, also known as a plosive or oral occlusive, is a consonant in which the vocal tract is blocked so that all airflow ceases.
This explanation is from here.