I am not a expert for urban planning, but I think the reasons are defective urban plan and developers.
This webiste is about the history of Xiyin urban planning.
According this website,
In 1985 to 1995, the goverment plan the limited heights of A1、A2、A4、A6、A7、A14、A15、A21、A22 are 85m and A8、A9、A10、A11、A16、A17 are 45m. Finally, A3、A5、A18、A23 have to be higher than 126m for Xiyin distruct. It is because the air route of Song Shan airport pass through this area.
It is totally different expectations compare with these pictures. You can see, A1, A2, A6, A7 is much higher than 85m.
The reason is A22, 23 (Taipei 101). The original plan is build a 50 floors tower. (Sorry, I can not find the rendering.) But during the proposal stage, it was added to 508m to make a world tallest building. It was a result of negotiation between Civil Aeronautics Administration and Taipei government. Air route of Song Shan aiport was changed and the limited high of whole Xiyin district became 600m. But, the limited height of A8、A9、A10、A11、A16、A17
are still 45m. The reason is there was a plenty of department stores had been built in these sites before Taipei 101 started to build. Eventually, Taipei government kept this area as low density development. Therefore, we can see a huge gap when we see the skyline.
One thing, I need to mention. B3, B4, B5 in picture above, they were built around 1980s to 1990s. And A1, A2, A3, A6, A9...etc, most of them were built after 2000. Between 1990 to 2000, the development of Xiyin District was not going well because the development cost, economic issue and location. The developers were not interesting in that time. Government do a lot of thing to encourage developers, but still useless. And you can see the buildings were built after 2000 is much better than these which built around 1980s to 1990s.
Why skyline of Xiyin district is so distorted? I think it is because it takes too much time to develop and defective urban plan that set by Taipei government.