Scala and Java are reasonably well served by traditional IDEs. Dynamic languages (such as ruby and python) are not, and based on the presentations, that's the market he is trying to serve. As for the choice between Ruby and Python, they are sufficiently similar that most of the work done to make one work will help with the other, and the choice mostly is which one he wants to support first. Python has a bigger community.
I mostly agree with you: Using IntelliJ (Java) and Rubymine (Ruby) is a great experience, however PyCharm (Python) provides the same nice dev experience.
All that said, I contributed fairly early on, and I wish that they would concentrate on just Clojure. The reason for my preference is that the LT devs are going to be exploring new ways to code and experimenting with just one language seems sufficient.
They actually opened it up to additional funding specifically to include Python. That money was raised based on community support (including financial support), so now they're including it.
Why are you so surprised that there'd be support/choices for Python?
Python's a great fit, certainly a more straight-forward one than java/scala. Presumably python/ruby would be equally interesting additions to Light Table
More people program in Python so they have a larger market to target than Ruby or Scala. If you're trying to start a business that's probably the best way to choose. I would guess that Ruby would soon follow.
I think it would be nice if the linked kickstarter page (which has many "why this" and "what about that" answers) explained why they chose Python. I'm pretty baffled. Just because it's popular? Better to pick C# then. Because it's... er, I can't even think of another reason.
Let's consider the "obvious" choices by examing the TIOBE top 10:
1. C - Not really the target audience of a tool like this.
2. Java - See above, plus plenty of competition.
3. C++ - See C.
4. Objective-C - See C, also arguably a bit of a niche (iOS).
5. C# - See Java, VirtualStudio dominates.
6. PHP - Yeah, right.
7. (Visual) Basic - Who are you kidding?
8. Python - Oh, right.
9. Perl - Only ranks high because of sys admins.
10. JavaScript - Already covered.
11. Ruby - Like Python, see below.
Basically your question seems to boil down to either of these:
1. Why was Python picked over Java/C#?
2. Why was Python picked over Ruby?
As for #1: the answer should seem obvious if you consider the category the three languages (JS, Clojure and Python) share. Plus, the market is very different and the competition is very strong and established.
As for Ruby: It's probably personal preference. There really isn't any objective reason to pick Python over Ruby or vice versa. The two are more similar than either of them would like to admit and they are perfectly interchangeable in most situations.
