It rather depends what you want to do with your light twin hele, which makes certain characteristics more important than others. Horses for courses really. If you look at most of them, they are used in the police, paramilitary and HEMS role. Relatively few are used for public transport/corporate, although some are.
To answer your specific question about why it has sold so well, I think that it has been carefully focussed at the multi-unit market where major nations buy them in bulk for state operations. However, they probably have to cut the price for large orders.
The cabin layout is particularly suitable for the parapublic role - less so for comfy VIP transport. The ride is not wonderful in the back (it is much better with the autopilot fitted) when compared with other ships, for example the AS355. The rigid rotorhead is a limiting factor in that respect but has other advantages.
My experience wouldn't really support the "bad support" argument for the 135. I am not saying they or the support are perfect, but they compare favourably with much of the competition. No helicopter manufacturer provides support that even appraches that provided by a car manufacturer, for example.
The 135 did have problems in the early years - IMO, because EC is large and government backed, and because they sold so many, EC has been able to invest in fixes. By comparison with some other competitors, it is well sorted and most of the post-production development inevitably associated with civil helicopters has now been completed.
I personally like the glass cockpit and autopilot - a great help for SPIFR. However, it is my first experience of this generation of helicopter, so others may be as good.