I think the biggest factor is price. For people east to Bournemouth, you have London Gatwick. For people west to Bournemouth, you have Bristol. Therefore, although the distance is further. For a cheaper flight? I and many others I know do drive further for a cheaper flight.
Believe it or not, but I know a family who went to Manchester to get a very good deal on a holiday to Mexico!
Ryanair, Easyjet, or who ever comes in - must offer good flights, not to places where people will not want to go to. At an competitive price with good times. Some of the Ryanair offerings this year were at the most ridiculous of times and most silly of dates!
There is no reason why people wouldn't use Bournemouth Airport, there just simply isn't enough choice.
Maybe if Ryanair had a smaller plane they could get it to work. As 189 people is a big number from a small airport! Is there even enough seats for 189 in the departure lounge?