I don't quite understand the logic of locking pregnancy out of the dominant route. It's probably my only complaint of the game. I like both routes quite a lot but lean towards the dominant route simply because it offers a few more scenes for my favorite tag.
Uhm this is a difficult answer for me to give, so bear with me because it will be a little long ( not too much, I don't want to annoy you).
The first part of this post it's an uneducated guess of mine, based on what I read about the dominant route, so take it with a big grain of salt because I didn't played it ( I will explain why at the end).
The dominant route have two possible outcomes:
1 You take advantage of Bella's situation to make her your obediently slave/pet, it's the less dark one.
You still treat her good with a certain degree of respect, you don't abuse her physically nor mentally and at the end of the story she become yours, now this give you the possibility to use her sexually as much as you want, with whoever girl you want ( your harem/solo/multiple girls your choice);
2 You dominate her from the start and she become a slave pure and simple.
She's completely at your mercy and you can do to her whatever you want, she can't refuse anything, there's no respect no kindness, she must obey to each order you will give her regardless of her opinion, this the more darker path;
Now what could be the reason of no pregnancy? Well the reasons can be different but I think the first that comes to mind is you don't love her, at all.
How could you love someone that you downgraded to the level of a pet ( first hypothesis) or even less ( 2nd hypothesis) to the point to have a child with them? Wether you can debate on this she's not your equal, so why making her bearing a child that would be pretty much the same as her?
She's here to offer you relief sexually when the other girls are pregnant, how could she be pregnant then? it would be a bother and nothing else, she's there to serve you and the rest of your girls, not to be loved not to receive kindness ( well a little bit of that sure, but just to make her proud of being a good pet in the lighter path, or of being a good onanhole in the darker).
So at the end of the story a child with a Bella in this condition is really an option? Imho no, if anything else it would be only a disturbance from her.
And I write this in the most neutral way, when I use the word downgraded it is free of any bad/negative thoughts ( at least as much as possible, you can't never be completely neutral when offering your personal opinion).
Now why didn't I played the dominant route? Well it's the fact that to me it sounded redundant, I mean we've already had the biggest theme that was the incest, offered in a somewhat realistic ( again my personal opinion) take in respect of other games.
Having this theme to fully explore in all the different variations ( because all the girls in the family have a different character and arc to develop both in the harem and in the solo path) was enough for me that, having another big theme like that was too much to examine.
Also differently from other games I played, here the dominant route start not with you that slowly corrupt a girl ( or many) and transform her/them in a slave/pet, she's already in altered state of mind due to the sextortion.
Honestly I felt it was too much, especially after you clear the water with Bella, and you find out she's the most romantic and sweet of them all ( again personal opinion so....).
So yeah, that's what I think about the no pregnancy in the dominating route and why I personally think it's best only in the romantic route.
I have nothing against games that offer you the possibility to dominate a girl to the point to transform her in a slave ( Pale Carnation is one of my favourite games on this theme).
But differently from SL, PC already put you in that road, there's nothing else, in Sl instead you have the incest as a big theme and then that and....It didn't worked for me considering how the story unfolded, that's it.
Sorry for the wall of text, hope it wasn't too boring to read for you.
And if you want to continue the discussion we can always start a private conversation, I don't want to clog the thread.