First is the disclaimer: never, ever, ever apply genetics to fantasy races. That way madness lies.
Now, let's examine things a bit. Half-elves and half-orcs are humanoids. Their lineages are those of humanoids: humans, elves, orcs. No creature type changes.
Tieflings and aasimar have less than half-blood. Lineages for them let you trace back to specific subtypes (e.g. pitspawn), but they're outsiders either way. Unique races with mixed traits, true enough, but their own races.
Half-fiends and others are one parent humanoid, one parent outsider (or whatnot). Maybe they ended up with enough of the outsider parent to gain specific traits of that outsider (check out Demons Revisited for examples of half-demons with specific parents), maybe not (basic half-fiend).
Class bloodlines just tell you that you're descended from something, if even then. Even then you've got some trouble. Arcane bloodline, for example: you're descended from...magic? How'd that work? Imperial/destined bloodlines come to mind.
As for why creatures very seldom have two types: it can cause problems with a few rules interactions. Bane weapons, for example. Favored enemy ability also comes to mind. I personally don't always agree with the prioritization (I'd put outsiders over dragons, for example), but it's mostly to avoid annoying rules issues.
Do note that templates like the half-dragon change creature type. Sometimes though, subtypes hold true, telling you what lies beneath the skin.
Thank you Lathiira, your explanation gives me the reason as to why though I may not agree whole heartedly with that reason. I do see your point about outsider vs dragon priority though it would make more sense to me that way also. Still for it all to basically boil down to rules feels lacking to me though being a GM who prefers to follow the rules the best I can interpret them, this makes more sense. I can actually see the other side of the coin as well A.K.A. the players point of view when it comes to this kind of ruling.
Somehow seeing it written or basically hearing this makes me feel more at ease, when ever I have to tell a player that doesn't work cause such and such is not humanoid or that isn't technically a monster it is this.