[wsxcde]

The answer is because the script evolved and was not designed so letters appear in somewhat random order.

Brahmi, which is the parent script for most Indian scripts as well as a bunch of other scripts in the region (Tibetan, Thai, Burmese, Javanese etc.), has a well-designed alphabet ordering. Simple vowels come first, followed by dipthongs, followed by constants. Consonants themselves are ordered as velar first, followed by palatals, retroflexes, dental and finally labial consonants -- notice the articulation point is moving from the back to the front of the mouth. Within each group the ordering is unvoiced, unvoiced aspirated, voiced, voiced aspirated and finally nasal consonant. It's a thing of beauty.