Because going between any two languages is a separate task. Unless you have a universal language that contains every linguistic feature everywhere and can achieve lossless transition, using X->Y and X->Z to make Y->Z will introduce an unacceptable level of error. English is not a universal language.

My Natural Language Systems professor just went through that topic yesterday -- that in the case of all machine translation applications, the ultimate goal is to get as high up in a 'natural language pyramid' where the tip is a single universal language. So we get the most use out of elements of any two given languages that have as much in common with each other as possible; these common elements could then be applied to other similar languages.

Interestingly, he also proposed that the closest thing we have right now to that 'tip' language is sanskrit.

In the 80's the Wang corporation (of Wang wordprocessing fame) was working on using Aymara as such a bridge language. I don't know what happened to the project.

I think most people who are serious about learning a foreign language either want to learn English or already speak it. It's relatively rare for, say, a person who speaks only Spanish to start studying Mandarin without even basic fluency in English.

The content on memrise is by large contributed by users. I've created a Portuguese->Spanish course there, and so have other people.

Noone is stopping you from creating a French->Italian course. Add 10 words a day and soon you'll have a basic course.

Actually it does have a lot of x language->y language courses, when browsing courses you can select the base language on the left where it says 'For [English] speakers'. (Though it appears there isn't a french->italian course...)

I was interested in Italian>Whatever (sorry, I inverted them in my first post). There only seems to be Italian>Mandarin. Though luck...

Anyway thanks for the suggestion.