Probably to discourage reaction, to increase the signal to noise ratio.
In cases it's 100% necessary you can use asciiart or a link to imgur or something.
For one images would put even more strain on the server (servers?) that HN runs on. Also, I fear that it would reduce the quality of the discussion, as at least on reddit, image posts seem to lead to repetitive and unproductive discussion.
HN is all about sharing articles and talking about them. Images should be left to the articles. If in the rare case images are needed to get a point across, links to other sites (and existing image sharing services) suffice.
No, please no.
Rather than 'giving more context' images (or thumbnails FTM) would just distract from the subject given in title.
I'm 15YO on Reddit and I still use the <old.reddit> interface.
Images/cards on link aggregators are primarily about maximizing clickthrough rate and/or time on site, which HN does not need to do.