It’s because it never wants to give you the practicable version and it takes you so long to farm it that it causes extreme sadness, also known as heartbreak.
I’m also encountering a similar problem with the orphan maker, quest rewards just do not like me.
Welcome to quest reward farming… and Seraph vendor farming… and farming in general.
Random is Random. It is a salad of randomness. So we have RANDOM SALADS.
Welcome to the amazing world of RNG.
If quest reward farming had any semblance of fun, or took skill, I wouldn’t mind. But one day while I was trying to get an eviscerating slag Rubi, whilst behind on my homework, I asked myself if I was having fun, if this is why I’m playing instead of working. I decided to do no more than 10 tries for quest rewards, and the rest of this story is against forum rules.
This is sadly my feeling too.
I tried to get a good FotFH for DAYS. DAYS. And i failed miserably.
Running to Quest giver. Accept reward. Check reward. Dashboard. Repeat. Not fun. At all.
I stopped once i got 2 Maliwan parts on it, couldn’t bear more…
At least killing bosses or ennemies can be a bit entertaining…
I really hope that for the next Borderland game they will implement some kind of Weapon Crafting Machine that let you salvage component from unwanted weapons to use them to craft a new one.
Or even fuse two identical aprts to get one better.
2 White Dahl SMG stocks = 1 Green Dahl SMG stock
2 Green Dahl SMG stocks = 1 Blue Dahl SMG stock
2 Blue Dahl SMG stocks = 1 Purprle Dahl SMG stock
2 Purple Dahl SMG stocks = 1 Etech Dahl SMG stock
2 Etech Dahl SMG stocks = 1 Legendary Dahl SMG stock.
32 white Dahl SMG stocks to get is a bit grindy but just enough to be acceptable for me (have to find a SMG, with good lvl, with good stock). And you still have the grips and the sight and the accessory to grind to perfect your weapon. Farmers are happy, RNG haters are happy (i HATE random, at least with that process i KNOW i will get my item, even if it takes days).
And to avoid exploit, make them tied to lvl. A stock has the lvl of the weapon it comes from, and can not be fused with a different level stock.
If they had a way to upgrade specific parts on guns for a certain amount of eridium I think that would be a good system. Even if it was like 250 I think it would work well
Wait hold on a minute… the practible version of the heartbreaker is rare XD I just wanted one for my siren and got it first try.
Yeah, these things happen. YMMV and all that jazz.
In this case Im glad. Ive been using the thing forever. So good.
Just got this first try while turning in the mission. Lucky me?
Oh yeah. You should be glad. Took me forever to get that thing 
Could have sworn it was because Moxxi got the gun from Pat Benatar.