We can't explain it. Kana is a system of writing and pronunciation for the Japanese language which is similar to the alphabet systems of the european languages. Linguists have devised ways of categorizing language. There are two kinds of units of sound in a language. Phonemes are the smallest units which have distinctive meaning.
Syllables are the segmental units of spoken words which are sequences of one or more phonemes.
For Japanese some linguists have defined a third unit of sound, the mora. The mora represents a sequence of sounds that are percieved as being of equal length by the native speaker and are a unit of sound somewhere between a phoneme and a syllable. Due to the way in which the sounds of the Japanese language are patterned, consonants are, with a single exception, always expressed in combination with vowels, such that the vowel antecedes the consonant. From our point of view, doubled s, t, and k simply represent phonemical variants of the non-doubled consonants. There are three kinds of morae, should one be inclined to call a syllable, however peculiarly constituted, a mora.