There are enough Bug/Flying types already. And Butterfree has always had a connection to the Psychic type since it's been able to learn Confusion on an early level ever since Generation 1.
Orbeetle isn't in this game so there was still plenty of room for a Bug/Psychic type.

The Cubone doesn't really stand out stats and type wise. Since on those areas it's basically the same as Sandshrew and Phanpy.
Giving it the ghost type not only makes it more unique, but it also fits the Pokemon's lore.

Same thing as Cubone.

There are plenty Normal types. And in this Generation there are no Normal/Ground types yet. So in order to make Furret a bit more special it has been given the Ground type.
Pretty sure in one of the anime episodes Furret was actually called a Ground type too.

Sunflora is kinda boring.
Pure Grass, awful stats, simple design...

Giving it the Fire type makes it at least unique and somewhat interesting. It's a cool type combination and out of all Grass types Sunflora is one of if not the best candidate.

We don't have a Bug/Dragon type Pokemon yet. Because of this a lot of people say Yanmega should be one. And while this isn't a horrible idea. Yanmega is already a perfectly fine Pokemon with good stats.
Dustox on the other hand has absolutely nothing going for it. There are already other Bug/Poison types in Beedrill and Venomoth, and Dustox doesn't do much better than them.
Therefor Dustox is the Bug/Dragon in this game.

Surskit is a pretty unique Pokemon, being the only Bug/Water Pokemon until generation 7. But when it evolves it turns into Bug/Flying which is one of the most used type combinations in existence?
That's dumb so here Masquerain stays Bug/Water.

We have enough pure Normal types with high speed and nothing else.
Mega Lopunny is Normal/Fighting so why not regular Lopunny as well?

When I first saw a Pachirisu I thought it was an Ice type.
And we don't have any Electric/Ice types yet besides Rotom so that's enough reason for me.

Lumineon is one of the most forgettable Pokemon out there. Giving it a unique typing (or at least a unique typing in this game) makes it stand out more.
You could say maybe Dark or Psychic would be more fitting for its design. But those type combinations are already present. And Water/Ghost isn't unfitting. It's that one fish you forgot existed and died because you forgot to feed it.

The Fairy typing doesn't exist in this game. So Alolan Ninetales is just pure Ice.

Same thing as Alolan Ninetales.

There are enough Water/Ice types already; Dewgong, Lapras, Walrein.
There is also already an Ice/Flying type in Articuno of course. But that's a legendary only available very late into the game.
Delibird itself is also Ice/Flying so keeping it that way just seems more logical to me.