Again, you're only relating a small sample, and a skewed sample at that.
It's not really as skewed as you want to assert. The fact one store had a wall of Trek merch that went fast says a lot. It's not even anecdotal - it's research. We don't sell Trek merch because we're Trek fans - we sell it because people want it.
The Works didn't sell out of Trek merch because one person went in and bought everything - they sold out in days because people wanted it. People don't ask for stuff cash in hand because they want to test the waters - they go in because they want it.
You talked to the managers. They told you the merchandise isn't selling and doesn't have enough of a demand so they aren't carrying much of it.
The
staff of one store say that. The last time they did, a rival store bought in the merch that was being asked for and sold out quickly. Again, not as much a skewed result as a store saw a demand, sold out. Another store (the more high profile store) didn't see the value and someone else profited from the oversight.
Any time that particular store has Trek merch, it's sold fast - aside form overpriced items. Keyrings, phasers, even Rubies uniforms - they don't last long. The high end merchandise doesn't sell as fast, but thats whats available and thats whats lying on shelves and that causes the assumption there's no demand.
That is a skewed result. Collectors want to buy Diamond Select high end replica's. People want something cool for cheaps. The latter doesn't exist.
To be honest - I find more fault with the lack of even bothering to try with merch rather than the stoes not stocking anything. Aside from items that are decades out of date and highly priced, there's bugger all to actually buy in stores for the product thats out there. Aside from Asda which has hotwheels vans with William Shatners face on the side, which is just awful.
I don't know what cons you went to this year but I feel bad for you. The ones I went to were loaded with Trek merchandise and cast members. Since this was the 50th, Trek had the biggest floor space in the events of any franchise. All of it overpriced, I totally agree with you there, but it was everywhere.
We had one official Star Trek convention in the UK. The less said about that the better

But, despite the above, that could be an interesting factor to take into account. Of the two major conventions, the younger 'cosplay' market is focused on more now than ever and the guests from overseas aren't as accessible due to extra cost and expense.
As such the only major thing on show is aimed to that younger market, primarily teens. And most teens appreciate a YouTube star more than an anniversary (I'm sadly not even being snarky!

)
I personally think that the problem isn't lack of demand, but the mix of products that is out there right now. The one Star Trek product I saw at Target the other day was a remote control Ent-A model that looks really cool and retailed for $119 US. Which, ok, that's great, but not what I'm really looking for.
Thats the thing - The Ent A design is decades old and hasn't been on screen since the early 90's film focused on "we're old and grey."
The people who wanted the A bought it years ago in some shape or form. It's weird that a lot of the Trek items that come out are aimed at people who have been collecting variations of those items for a long time now.
Products for young female fans are an intriguing point of view - Star Wars had Rey, young girls had their icon and bought Rey. My daughter loves womens wrestling, she can go to a shop and buy an Alexa Bliss figure. Beyond brought a new kick ass female that can appeal well to young fans. But.... nothing.
It's a bizarre world where the BBC can sell figures of a grumpy old man, but Paramount live in fear of selling a young group of heroes :/