And open source is generally built either by volunteers (as I do part time) or paid staff (as I do full time) – but generally even in the case of the latter it is with a fraction of the level of investment that commercial vendors bring to bear on equivalent products, hence user experience can tend to suffer for a bit (certainly its probably the last bit that gets looked at).

Also – as different areas of software become “commoditised” its a great way for pay-for products to differentiate themselves – invest a lot in usability and it makes your product more likely to sell itself.

But I think its also a bit of a lost art – for some reason its seen as unsexy to care about user interface, or user experience compared with “hard core” parts (eg linux kernel developers are revered as having some magical power).

Furthermore, the web is to blame for a lot of this. In the 90′s .com goldrush graphic designers became web developers when following the money, but graphic design is only one small part of interface design and very little to do with user experience. However, the n-tiered approach to building apps meant that there was a “user interface layer” which became a detail to be worried about afterwards, (and often by people who would have been failed graphic designers in another life).