Lot's of reasons for this. Mostly having to do with the minimal size and sophistication of current quantum programs, and the background that many of these efforts arose from Physics teams where python is heavily used. One of the most popular non-Python based version of these sorts of frameworks is https://yaoquantum.org/ which is in Julia.

> minimal size and sophistication of current quantum programs,

For that, I'd expect a simple standalone like Verilog or ASM.

Well you also want to run the program and process the data, so for that later step it’s nice to stay in one language that has support for the data analysis.

Q# originated as a DSL in F#, which has extremely strong DSL-creating capabilities in its [computation expressions](https://docs.microsoft.com/en-us/dotnet/fsharp/language-refe...). See https://devblogs.microsoft.com/qsharp/why-do-we-need-q/ for its history.