Why scratching, not biting was the vector of infection in iZombie?

And if so, someone could get infected by sharing a coffee cup or even by hand-to-mouth contact with the same surfaces - something that was completely handwaved in the series.

You're overgeneralizing. You can't lump all "contact" disease vectors into one. Generally speaking, bloodborne pathogens spread by blood-to-blood contact. Because the zombie virus often spreads via biting, saliva-to-blood is relevant too. But the key thing here is ...-to-blood, i.e. the victim needs to get it in their bloodstream to be infected.

Sharing a coffeecup does not equate to blood to blood contact, at least not inherently. But say you have a cut on your lip, so you do leave trace amounts of blood on the coffee cup, and the other person also has a cut on their lip so the trace amounts of blood can enter their bloodstream, that is a notable disease vector.

There's no point in arguing the specifics of what constitutes a reasonable transmission vector as this varies per disease and we don't have specifics about fictional diseases anyway. For all we know, the zombie virus could be an STD as well but I'm not willing to risk the NSFW search results to look it up.

In regards to the "scratching also spreads the zombie virus" trope, the general approach is that it is only transferred via open cuts (i.e. direct access to the person's bloodstream), not via a surface scratch

hand-to-mouth contact

The mouth is not the issue, it's the bloodstream. If you have a cut in your mouth, your mouth is a danger zone because it has direct access to your bloodstream.

Without any unusual wounds in your body, this is not an issue. The first real access to the bloodstream in a healthy body is via the bowels, where nutrients are absorped into the bloodstream. But to get there, you have to pass the stomach. Stomach acid tends to disinfect most things. That is the primary purpose of stomach acid, along with (preparing the ingested contents for digestion).

This is the body working as intended. The stomach disinfects ingested contents. This prevents you from getting infected by unwanted bacteria and viruses. This applies to any pathogens, not just specifically the zombie virus.

Of course our stomachs aren't perfect and can let things slip through. Whether our stomachs are able to negate the zombie virus is up to the story writers to decide for their own story. Given the tendency for requiring blood contact, it implies that the stomachacid should negate the virus.

As an aside, the reason zombie bites are generally considered dangerous is because in those canons the virus is also present in the zombie's saliva. But generally speaking, the virus needs to enter the victims bloodstream in order for them to be infected.

I'm not telling you you should share a coffee cup with a zombie (or french kiss them), but without direct blood contact that bypasses the stomach, the infection chance of pseudoscientific variants of the zombie virus should be low to none.

But ignoring this fact - why the creators decided to change such well-known canon?

Simply put, it raises the danger factor. If only transmitted through biting, zombies become relatively easy to protect against compared to when any brief touch can be the killing blow.

But ignoring this fact - why the creators decided to change such well-known canon?

The zombie mutation does not follow a singular definition across all movies.

As a simple example, in The Walking Dead anyone who dies becomes a zombie, even if they've never come into contact with a zombie. That's atypical of other movies, but it does apply here.

In other cases, zombies are raised through necromancy (e.g. Michael Jackson's thriller), yet in other stories the origins are biological (whether engineered or natural).

You cannot lump these all together. The zombie trope is not as specific as you're taking it to be.

Why scratching instead of biting?

I am not aware of any story in which it is spread through scratching instead of biting. Whenever scratching is a danger, biting is still dangerous as well.

It's simply the case that in order to bite you, slow zombies (which seem to be the predominant trope but exceptions do exist) generally need to hold you down, which entails getting scratched. If that already transmits the virus, then the bite itself is irrelevant and thus not necessarily worth focusing on.

It also allows for uncertainty. A bite is definitive, but was this scratch enough to transmit the virus? The odds of infection for a light scratch are not as black and white as they are for a bite, which allows for plots where the victim doesn't know whether they're going to turn into a zombie or not.