thats generally because bombers are the counter to structures.. back in 'the day', the tutorial taught you to engage special forces posts with only frigates, because those outrange it, allowing for the destruction without loss.
Nowadays, there are virtually no structures (other than random guard posts) that are NOT countered by bombers - thus, bombers are considerably more useful of the fleet ships.
However, starships infringe on this fairly well - the bomber and siege starships fill virtually the SAME role, and are often seen as doing it better due to attrition.