Q. Why do devotees in ISKCON spell Krsna and not Krishna? What is the technical reason behind this?
A. It is not just Krsna. It has three dots below the alphabets ‘r’, ‘s’ and ‘n’. This system of using “Diacritic” marks to represent Sanskrit syllables using familiar Roman alphabets with minor modifications has many advantages.The problem arises because certain sounds in Sanskrit have no equivalents in the Roman alphabet and certain Roman alphabets have more than one pronunciation in English. To solve this problem, Srila Prabhupada used the diacritic marks in all his books. Standard authoritative books used by the academia always use these marks. So, with these marks in his books, the  books would gain acceptance from the scholars and reserachers as standard  textbooks for studying Vedic culture.

Also, this practice enabled the  western devotees unfamiliar with the pronunciation of Sanskrit syllables  to  pronounce each Sanskrit syllable properly and receive the benefit of  correctly chanting the Vedic mantras and verses from Vedic literature like  Bhagavad-gita and Srimad Bhagavatam etc. originally composed in Sanskrit. In  books written by Srila Prabhupada, you will find many such words that may  appear to be different, but on close inspection will tell you that they represent correct pronunciation of the original Sanskrit text.