Roy Sullivan, a park ranger in Virginia, claims to have been struck by lightning 7 times. Does that mean that being struck by lightning happens to 25% of the population? Of course not. He was just extremely unlucky.
So yes, some people may have well have had a string of bad DMs. But in that poll we have no idea how many DMs the respondents have had or why they considered them bad. For some people "bad" may mean they were not allowed to play an evil character. For others the DMs were bad because the DM was an abusive a-hole who couldn't retain a group and only ran a few games and the person answering the survey happened to be unlucky enough to play with that DM for the brief period of time they were DMing.
But a bad DM to me means, among other things, that you don't enjoy playing the game. So no, I don't think the game would be as popular as it is if 25% of people were not having fun playing. I have no idea where you get the number "people only play for a year" from. A fair number of people try it out, many don't have time to play because they have other time constraints or simply find that the game isn't for them.
But unlike Lake Woebegon where all the kids are above average of course most DMs will be average, and many will be starting out and make mistakes, I just don't think that makes them bad DMs even if they aren't running a game I want to play.