Why shouldn't I choose my back-end language randomly?
Ruby, Java, PHP, Python, C++, there are a multitude of languages.
Most of the differences between them are irrelevant when you launch your startup.
My advice: choose a well adapted language from the field of industry you aim for: general web/e-commerce - NodeJS/TypeScript, automotive/industry/embedded - C++, finance - .Net/C#/F#, Science: Python
I suspect you are wrong, and that the differences are relevant (and the optimum choice or set of choices depends on a lot on what specifically you are doing), but if you think the value of getting the choice right is outweighed by the cost of evaluation, or if you think there is no difference, go ahead and choose randomly. It's your (and your investors', if any) money, not mine, and you should definitely do what you think is right.
- speed of iteration -> how quickly can you produce something that proves the value
- available developers -> how many people code in this language and what kind of people are they
- infrastructure -> depending on what you plan to do, it might be a good choice to have something that is easily deployed to a cloud service, some languages are better supported than others