KDE mainly has a a lack of decision making by the devs, & a lack of taste: the default theming, icons & panels are all fugly, and it takes a world of configuration work to make it look reasonable.
The appearance options are so complicated and multi-layered that it's hard to replicate simple changes, and even installing a pre-built theme isn't wholesale: Separate types of themes need to be applied in a few different places.
Then you finally have the problem of GTK apps: They look bad in 99% of KDE themes, and most users are using at least some GTK apps (Firefox, Chrome ).
Also, I am not sure why you say Firefox looks ugly in KDE - http://album.gnufied.org/firefox.png . I have made 0 modifications in KDE/GTK theming and firefox looks just fine under KDE.
The appearance options are so complicated and multi-layered that it's hard to replicate simple changes, and even installing a pre-built theme isn't wholesale: Separate types of themes need to be applied in a few different places.
Then you finally have the problem of GTK apps: They look bad in 99% of KDE themes, and most users are using at least some GTK apps (Firefox, Chrome ).