Because they can't find anyone they like, they are stuck making bad/deperate hires or with the prospect of rebuilding the codebase. The list of technologies that are tough to hire for vary by market. In my midwest market, Rails still falls in that group and I know a ton of companies that can't find the talent they need. Do your best to stick with the technology stack that has the largest candidate pool in your market.
There might not be as many Clojure programmers as Java Programmers, but there's probably a higher proportion of good programmers working in Clojure than Java. Hire whoever you can find who is technically able, passionate about tech and your idea, and I'd wager they will pick it up pretty damn quick.
I would say that though it's easier to find a Java programmer than a Python programmer, it's easier to find a _competent_ Python programmer than a competent Java programmer.
Java is NOT the only option.
If you think you can find people to work for your company who either know those languages or can learn them quickly enough to not kill your productivity or your product, then go for it. Otherwise, you may want to think about making some tradeoffs (i.e. node.js over Erlang because you're going to be far more likely to find competent js devs than Erlang devs.)
Most acquisitions end up with the founding team contracted to transition for a year or three; during that time the acquiring company either rewrites the IP or ends up with enough qualified people to take it over.
When and if (and it's a BIG if) companies are expressing interest in acquiring your technology, you can sort out whether you'll need to rewrite portions as part of a deal. But far more likely is that the product will pivot at least to some degree, and you'll be thankful that you're using technology that you can comfortably and quickly modify.
I'm not sure there is a huge productivity difference between the modern frameworks unless you really know nothing about what you're getting into. I'd choose something mainstream that you all know at least a little and can use and then get to work.