asynchronous downloading

[Plumo]

I want to download content asynchronously. This would be straightforward to do threaded or across processes, but difficult asynchronously so people seem to rely on external libraries (twisted / gevent / eventlet). 

(I would use gevent under different circumstances, but currently need to stick to standard libraries.)

I looked around and found there is little interest in developing a proper HTTP client on asyncore. The best I found stopped development a decade ago: http://sourceforge.net/projects/asynchttp/ 

What do you recommend? 
And why is there poor support for asynchronous execution?

Richard