Because in Hiragana vowels are completely different characters than consonants and not symbols [vowels: い あ, consonants: し, じ] and consonant-vowel mixing [S+O, S+A] will create a new character [so, saそ さ]. Thats the idea of vowels and consonants. To keep them separate through an alphabet. This can be easily done to Hindi and any 21 Indian languages and I have done this through “indougana” a mapping of Indian alphabet to Hiragana [in 30 minutes].
Note: there could be exceptions: eg there could be some missing characters which are not necessary. SO I am not saying replace one language with another but study them together and make one better if there is really something that can be brought from another language. My senses say Indian languages need much more modernization at present the first of which should come from unifying all 21 alphabets into just one. DOn’t even say which one. Ask me and I will develop one. Indougana is a 1st step towards this. One can simply take about two consonants from each Indian language [part of it] to develop one unified alphabet system. Vowels are 5 in number hence one can simply retain 5 provinces. [or kshiti ap, tej, marut, byom].