When I was in the USAF, you became part of a bomber crew when you didn't get your 1st choice to be a fighter pilot.
That is still predominantly the case. When my FTU initial class got asked by the academic instructor: "raise your hand if you placed this assignment #1 on your dream sheet", not a single soul raised their hand.
Crap, somebody pour me a straight one, this is gonna be a long one....
There are other logistical reasons why bombers exist in the T-38 track; most of it has to do with creating a repository for fighter assignment re-tracks. Specifically, those who do not qualify medically due to failing the centrifuge profile for their fighter airframe (the Viper one is the one that gets most people) or those who get diagnosed with a "single-seat disqualifying" incapacitation-risk condition, get sent to bombers. The second group you're looking at is IFF or RTU/FTU washouts. The third tier of unlucky winners are FAIPs finishing their instructor tour in white jets who end up facing a follow-on assignment drop just as devoid of fighter slots as their original pilot training class assignment night was. Also, if they ****ed someone off during their tenure in AETC (don't slap the IFF SQ/CC at the bar while a T-6 FAIP everybody, or you too will get Buffs two years down the road...), it is understood that some of these follow-on assignments are punitive in spirit. You'll never be able to prove that of course. In any event, timing and luck is the Hunter in this merry go round of crapshoots. It's a real trail of tears to bombers usually. And it's like Shawshank too; we're all innocent here, know what I mean?
As to my own sob story: I became a Reserve bomber pilot because economic need eventually won out over getting hired by a Guard Viper unit to fly my dream airplane. I got a couple interviews, but never the job. My application process spanned the better part of 3 years and a couple dozen (some repeat) applications in that time frame, while concurrently pursuing engineering graduate school in order to justify my livelihood (I never told the University that, of course) and theoretically make myself more competitive to the hiring units.
The gamble lost. My education and FAA self-funded tickets to CFI/I didn't help me one iota break into the gig. I fell short in the networking department. So, 8 years of wasted effort. Eventually the school job and the college holding pattern EFC ran out. I just couldn't continue playing russian roulette with getting into USAF pilot training; there is an age cutoff after all and I didn't want to be a 29 yo 2nd LT. So, gave up the dream and danced with whoever brung me. Got hired by a bunch of heavy units. Bruised ego in tow (my sentiments at the time), I picked the one who would at least send me to 38s and keep the door open (bombers, and no, it doesn't keep any doors open btw). Had a freggin' blast in 38s and did well too, not that it matters for a Guard/Res guy going to heavies. The fun factor went downhill from there, as I expected. The rest is history. If you were ever curious why my screen name is what it is, this little story is why.
Not all was lost though. Fast forward 7ish years and life is better these days. I manged to escape that community to a gig on the trainer side, got divorced, got a more military-friendly wife in the process, and I'm much happier with the kind of flying I do now (aerobatics, Gs, and formation) Landed a much needed full time AGR job. Had a healthy baby boy. So we're blessed.
I'll probably leave this job when they fire me, which trust me they're trying to get rid of these "cadillac jobs" in a bad way. I once heard someone say, "do something until it stops being fun or you get fired". Figured what the hell, I'll follow that advice with my so-called military "career". As a father now, I can certainly assert that it could be much worse. So I still count my blessings. Who knows, if United or AA want to cold call me one of these days I may just put those 1000 odd hours in that old POS to good use. Layovers to the Caribbean only boys, and no commute to domicile for this guy
Sorry for the novel, it has been quite cathartic for me.
--break break--
For clarification to the poster who said you have to be the rank of Capt (O-3) in order to be an Aircraft Commander in bombers. That's usually how it plays out training delay-wise in Active Duty, but it's not at all driven by any regulation. Case in point: I was a combat mission ready B-52H Aircraft Commander at the rank of 1st LT (O-2).