The simple answer is that people just don't know about it.
They were popular gambling devices early on, then later outlawed. Pins were found in bars, laundromats, and theatres and then they started disappearing from those areas. There was a resurgence of them in the 70s with "Tommy " and marketing via Elton John. The 80s saw an arcade boom and the early 90s saw a renewed interest as well. Pinball crashed in the late 90s, and it has only been in the last few years that it has seen a regeneration of interest due to nostalgia, arcades, and new pinball companies producing machines. Unfortunately, many of those that remembered playing have passed away or are too old to care. Those that grew up with them in the 70s and 80s make up most of the collectors/players today, and unless we pass it on...it will be forgotten.
It is very common for me to encounter someone that has no clue what a pinball is, or that gives me the "I thought they stopped making those in the 70s" line. Those that do know what they are often have no clue there are rules/goals to pinballs...they simply think you are batting a ball around randomly.
If we want this hobby to become more relevant and grow, it is up to all pinheads to share the hobby via talking about it, inviting people to play our games, explaining actual rules and play styles, opening arcades, bringing people to shows, and buying games.