Why are wine bottle volumes in centiliters not in liters or milliliters?
Part of the answer is in the marketing system used in a particular country or region and part of it would be about the size of bottle being purchased.
In Canada and the USA, in a standard bottle of wine, there are 750 millilitres (ml), 75 centilitres (cl) or 0.75 litres (l). Wine bottles aren't quite litre-size, but the average wine bottle will contain 750ml of wine.
I see only 750ml bottles in the area where I live. I am fine with that. A bottle of wine marketed as 0.75 liters seems less appealing than wine sold in a bottle marketed as 750ml. And one marketed with a 75cl label seems less traditional in this neck of the world and may not sell as good as one sold with 750ml marked on the label. It is sold in a more positive light so to speak. It is all marketing.
I am sure there are other reasons, but take a look at Difford's Guide to mandatory bottle sizes around the world. It is an eye opener to say the least.
However there is more to the Subject:
When glass bottles become popular though, it is surprising to see that both continental Europe and Great Britain had glass containers of a similar size. The British had an official glass-container size of a fifth of their Imperial (UK) Gallon at around 900ml, while other Europeans' bottles gravitated around 700ml to 800ml. The US officially adopted the 750ml bottle as a standard in 1979, as a metric equivalent to the fifth of a US Gallon (757ml). - Standard Bottle
Alcohol measurement in bottles has undergone several revisions in the last few decades as Difford's Guide point out for the USA mandatory spirits bottle sizes (January 1, 1980) and the European Union mandatory spirits bottle sizes (January 1, 1990).