I have a theory (but no idea how to test it!) “A Q&A site that is free, quick, relatively reliable, little advertising, does not demand personal details etc? This is not the real world!” – at which point logical reference points fly out the window and the fantasy of a fairy godmother (who can fix everything) takes over
But it is also some practical considerations. If English is not the OP’s first language the message received will not be as clear as otherwise and there is a lot less incentive to read the details of what is and what is not on topic, etc. (May account for example for a question on SO about where to buy a particular brand of cigarettes.)
Site confusion is still a possibility. The design aspects you mention help to make the context clear but lose some of that impact for anyone “at home” at more than one SE site. I know this is the case as I flit between two or three and on one recently the OP readily admitted it was purely his mistake as he had never intended to post where he did. (If you have a problem, don’t really know what it is, are struggling with how to express it and with formatting and trying to comply with all the various conventions all at once your mind might be distracted – the style is familiar, but for the wrong reason!)
More seriously, in my view, but perhaps convenient that it is the most under Travel.SE’s control, is the “normal” in “normal off-topic questions”. I have not noticed
many programming questions here (I’d be interested in an example) but certainly have seen a few about cellphones. Are these really not-normal off-topic questions? At present “cellphone closed:0” returns 14 results with around net +85 votes between them and ~30 answers. “mobile closed:0” 85 results, though at least a couple of these do not concern telephony and I’d expect for some any comms aspect is more incidental than core.
Travel.SE is clearly distinct in nature from the Trilogy, where technical questions and answers are more easily categorised. Answers in particular tend to be either right or wrong and there is little “grey”. (So, thankfully, less scope for differences of opinion and where emotions do not run as high.) The Trilogy of course has had longer to reach something of a consensus about what is and what is not on topic. Travel.SE is clearly a long way short of that at present. You have your idea of what is related to and not related to travel here but others quite often do not agree with you. “totally unrelated” should be easier to agree on but I suggest it is part of the same continuum, just crops up less frequently but will still sometimes do so.
the reason behind this?
No one thing, there are several.
we are just used to the site
Possibly the main ‘problem’. You are convinced it works one way, others are equally convinced it works another (slightly different). Some at least of the people who “ask questions totally unrelated to travel” are, in my opinion, being sent mixed messages as a result and not reading them the way you think they should.
I think that is part of it, in particular where questions may be on topic or close to being on topic at Aviation or Expatriates or The Great Outdoors.
* This question might be an example of belief in supernatural powers for TSE: “I am going to Germanh and taking a box with a chair in it. how much extra would I pay, and is it allowable”. Apart from a Title and [luggage] tag that’s all, no other details provided whatsoever.