*The airlines prevent them or: the airlines won't buy them if they were offered*

The photos you have already show an APU in the tail section (exhaust on the starboard side).

Adding air brakes requires structural integrity (weight; money) and additional system connections (weight; money).

When they are of no use, they're simply not added. Why are they of no use?

Well, pure air brakes (as opposed to air brakes / spoilers) have 1–2 functions (for jetliners):

1. Compensating for the lack of thrust reverses
2. Performing steep approaches when the thrust-to-weight ratio is high (small plane that has too much power). As I noted [here][1]: The thrust-to-weight ratio for the RJ100 is 0.28:1, compared to 0.16:1 for the comparable Boeing 717. It's even higher for the smaller RJ's.

Neither affect most jet-liners (note that the baby A318 is certified for steep approaches, and it doesn't need such an air brake).

The airlines would rather carry payload (paying load) and not bother paying for maintenance to regularly check and fix a novel item (not all airports are London City ;)).


  [1]: https://aviation.stackexchange.com/a/37413/14897