I'd say the main issue is one of opportunity.
Even if I can earn Cr1m from exploding an outlaw, that's still a lot of faff (travelling around, accepting missions and then more travelling to locate an outlaw) for a million credits compared to jumping into an AspX and spending 30 minutes mining 64t of stuff that I can sell for ~Cr12m.
I'd suggest there should be more involved combat missions, which provide you with more engaging gameplay and result in larger rewards.
For example, a faction would offer you a mission such as "Pirates have been ravaging our system. Wipe them out and we'll pay you Cr100m".
That'd give the player an hour or two of gameplay, it'd provide a bunch of targets and, of course, the reward would be substantial enough to make it feel worthwhile.
Also, it'd create a bit more of a strategic element than massacre missions since a smart BHer would need to locate systems that had sufficient Orbital Installations, Stations and RES's to generate lots of pirates.
By way of analogy, it kind of reminds me of earning gold in Skyrim.
You can earn a lot of gold simply by dungeon-diving and looting urns and chests, picking up 10 gold here and there.
It mounts up, eventually, to a substantial sum.
Alternatively, you can take a quest, complete it and be paid 1000 gold all at once... and still collect the gold you find along the way.
Combat in ED is kind of like dungeon-diving in Skyrim.
If you do it enough you will earn a reasonable amount of credits but it means you're "scraping around", earning a few credits here and a few credits there.
ED needs more stuff with big payouts so you can feel like you're doing something cohesive in return for a significant reward.