Because the AX is type approved for training, has a UK source of spares, a coherent flight manual, and stall characteristics that are compliant with the airworthiness standard? (Or the X'Air all but the type approval). Plus you're dealing with the BMAA, not the LAA - which in my experience is massively more user friendly if you want any mods done.
Chillindan - the running cost of a 2 seat aircraft with a Permit to Fly will be a few hundred a year more than a deregulated single seater - but for that you get airworthiness oversight and a passenger seat. Personally whilst I enjoy a bit of single seat flying occasionally, I like to take my friends flying more. Whether you're competent to look after an aircraft without independent oversight is a question worth asking yourself.
If you do fancy a deregulated single seater however - the Sherwood Kub has an excellent history and current product support. I'd also look at the Chaser or the Minimax, which by all accounts are great to fly and whilst the Minimax doesn't have a UK manufacturer, it has a really good owners community who can support you.
No idea about the Merlin 100, but it's certainly pretty.
G