Because the experience would be really bad for them. Say they turn up to a track meet. They enter the 800m. There might not be many entrants. Imagine there are six people. They start to warm up but probably feel intimidated. All of their training has been either on the road, grass, pavement or little trails. They see people prancing about kicking the air. People are wearing things on their arms (arm warmers), they have spikes on, they are in gym knickers (bun huggers). I suppose people wear these in road races but it's not as common. There is more variety in road races - people in vest tops, T shirts, long sleeved T shirts, maybe even tracksuit bottoms, tights, leggings and so on.
They look around nervously. There are lots of new rules to follow, etiquette. They get to the start. Everyone is looking! It's in a stadium and they are stunned by the lack of people. It feels so empty on the start! Just a few people and these other people appear to be sprinting very fast up and down the track and then stopping. They are told to go to their line. BANG! The gun goes off. Everyone flies away... sprinting as if they're running for dear life! Trot trot trot... the runner stumbles around the bend in her trusty Nike Pegasus. The other five people are way ahead down the track. They block it all out and carry on. Whoops! There's some kind of flag on the floor. The runner kicks it out of the way and moves on, realising they are in lane five and are now allowed to be in the inside lane at long last! The runners disappear into the horizon. Huff huff huff... she runs past a group of spectators who do a little clap for her.
At 400m, a loud bell sounds. By this point the other runners are nowhere to be seen. She carries on, embarrassed, humiliated almost, past more spectators, feeling like all eyes are on her. Voices boom from the PA system. She feels like all eyes are on her - the stadium is built up on one side and she gallops past everyone in the vicinity. It feels like everyone in a mile square can see her! Thank goodness, she's back to the back straight. Familiar territory and far away from the scary crowd on the other side of the stadium. She concentrates on her running. The final bend... people clap but she can't tell if they're being sarcastic or not. They're being nice but a sympathy clap is the worst thing she could hear right now! She just wants to disappear into the floor. Bursting across the line, she falls onto the floor, panting and red-faced. 2:42!!! The man announces. It's below her goal but she doesn't have that same glow, the sense of achievement. She just feels silly! The other runners have disappeared, walking off the track into the small crowd. One stays behind and shakes her hand, says well done. She just felt it was so sparse when she was running.
There is no medal, no memento, no T shirt, no cushy drink, no food. She grabs a bottle of water from the little hatch and pays £2.25 for it. What a bargain! The other runners all seem to know each other and are dressed up like superheroes in slinky lycra and they gather together to run around the track to cool down. Their cool down seems faster than her race pace! There is no one finishing around her, no one finishing behind her, there was no one for her to race really. She is embarrassed and quickly tries to leave before anyone talks to her, nice as the people seem to be.
The next day she tries to log onto the site to access her results but there don't seem to be any results anywhere. The next weekend she trots along to her local parkrun where she has more of a social event and finishes with runners all around her, hidden in the crowd.
This is why I feel people don't turn up to track races. It's not the fault of the meet organisers. It's a simple fact that if there are 100x less people in the race, people will feel more self conscious and I think that's what's affecting the slower runners and putting them off. Hobby joggers would not enjoy this experience IMO.