Why decline stage come in the product life cycle of product?

Products reach a decline point for various reasons but one example would be the typewriter. When first introduced, it represented a huge step forward in terms of producing written documents for the mass market and became very popular around the world. With the arrival of desk top computers and home printers at affordable prices, these began to become mass market products which could offer many advantages over typewriters such as document management, spell-checking and variable print outputs leading to a decline in the popularity of typewriters. Whilst there remain those who still prefer their trusty typewriter, they are a small and dying market and the premium price now required in such a small market, the end is clearly in sight. The same principle applies to most products. If a competitor introduces a faster, cheaper, more fashionable design, sales can decline rapidly and may never recover without innovative counter measures such as product redesign and relaunch.