The question of voice acting with regards to Katawa Shoujo comes up every now and again, so the decision to not have voice acting in the game bears some explanation.
Many of the decisions taken during the development of the game can be ascribed to lack of forethought and planning, staff coming and going, and the script having been worked on during much of the time all other areas were stalled. This has led to multiple problems, not the least of which has been a huge backlog for any artist joining the team, as well as offers having to be refused for various things to be included in the game. One of those is voice acting.
Time limits, with us wanting to have release dictated by when the art is done instead of adding yet another delay is probably the largest of the problems that voice acting faces. There is also the need to find a good voice actor for each and every character, and any written dialogue that's been recorded having to go unchanged and therefore not allowing for dialogue edits during post-production, editor sweeps, and alpha/beta tester feedback.
Finally, for a project the size of KS, the logistics of voice acting just become too much to work with. With the cast having grown considerably from the five heroines and protagonist that were started with, communication with the voice actors for each and every character becomes extremely difficult - a problem that timezones would also exacerbate.
For a smaller project, or one that is better planned, voice acting may well be a possibility. But for Katawa Shoujo, the decision has been made that it simply isn't.