It is the opposite of what you would expect. Around AD 700 the Norse language was changing fast, and gained more sounds to keep track of, not fewer. Even so, the writers threw out eight signs.
The result is an alphabet in which one rune has to cover several sounds: ᚴ writes both k and g, ᛏ both t and d. Reading a runic inscription is therefore always partly interpretation: you need to know the language to tell which sound is meant.
Nobody knows for sure why. The most convincing explanation is that the runes were not something you looked up, but something you knew by heart. Sixteen signs can be memorised. Twenty-four is a book.
Most rune material online still uses the 24-sign Elder Futhark, and often adds meanings that were invented in the twentieth century. We stick to the sixteen runes that were actually carved in Norway in the Viking Age, and to what the sources say about them.