Let's take the は ひ ふ へ ほ (ha hi hu/fu he ho) kana for example. When they get dakuten, they become ば び ぶ べ ぼ (ba bi bu be bo). The た ち つ て と (ta chi tsu te to) kana become だ ぢ づ で ど (da dji dzu de do).
Why this transformation happens. And, more important, why the ち (chi) & つ (tsu) are like this? They could have been ti, not chi and tu, not tsu. The ち & つ kana could also be categorized in the わ & ん category. If they were ti and tu, they could easily become di and du with dakuten.
So why does this happen? Why aren't they ti and tu instead of chi and tsu?