Part of it is games I think. When I was a teenager I was absolutely in love with M4s because of counter strike. Although they are very common across all media where guns are featured. When I first got into airsoft everyone was going crazy for the SPAS 12 and I think it was partly due to it being featured in the matrix, along with stuff like the MP5k. If the hobby was bigger I think you'd see the range of guns on offer grow organically and I doubt in the scheme of things if there are many, if any manufacturers that really care what the UK market wants.
Variation is nice but I always roll my eyes when someone says they don't want an M4 like everyone else then goes and buys a G36, MP5, M14 or to a lesser extent an AK variant which are all very common. If someone wants a certain model of gun then that's the best thing to advise them on, unless the only manufacturers producing said gun are terrible. Recommending a gun on the potentially false premise that it is almost unique isn't going to do anyone any favours in most cases.
It's quite interesting how much faith is put in essentially anonymous people online pretty much across the board. For sake of argument we could all say what we want with little recourse whereas if the chap down the local shop misleads you then it's much easier to give him a good talking to(or worse if you are that way inclined lol)
Whilst smaller communities like this one more or less police themselves, whilst the product reviews sections on online stores and on larger forums it's a bit like the wild west. I've repeatedly seen people slagging off products to the point where I question if they are just parroting something they heard someone say when you own said product and know a lot of people who do also that have nothing but good things to say about it. I won't mention anything specific as I think it would potentially lead to a massive thread derailment.