You're right, it does seem strange. It's probably the usual explanation, 'commercial reasons'.
I mean, Airfix has a strong British identity, and has offered a lot of great British subjects in various scales since they 'came back'. And a huge amount of aviation came from this country during WWI. Like you, I expected a few more subjects to be kitted after the E.II and the BE.2. More than that, they've now made a strong entry into 1/48, a scale still favoured by many Great War modellers. But no hint of a show there - I agree, a Camel could have been a clever pick.
And if we consider what Airfix might think of as their showpiece scale, 1/24, well, the field is wide open there. Not perhaps for a big range of kits, but certainly for two or three iconic types. It's not as if the UK is short of excellent examples of WWI planes, for Airfix to go and aim their LIDAR at!
So, those commercial reasons presumably amount to, 'there's no point in getting into Great War kits, because people only want WNW'. I think that's wrong, but I accept that other views can be held. ...Now, I'm fantasising about a glorious little grouping of 1/24 scale kits - a Camel, an Albatros D.III, a SPAD XIII, and a D.VII. Oh alright, and a Dr.I. I'm not in that particular fan club myself, but I know it'd sell
Mark