Aside from the images from even the richest inner city areas of Taiwan, which often look like the slums commonly found in the inner-cities of more developed countries, I'd imagine that government behaviour, including corruption akin to that found in the developing world, strange land use practices and the behaviour or Taiwanese citizens (from piling old scooters and garbage outside their houses to the archaic day to day religious or spiritual practices that seem to govern a large percentage of the populace) enhance the perception that Taiwan is a poor country.
Taiwan's location, next to China and just above the Philippines, and the fact that it sounds like 'Thailand' (so it therefore, subconsciously perhaps, must be the same as Thailand) probably does little to further its image as a country which is not poor, but in fact quite well off. Ironically, Taiwan was the butt of jokes as a developing nation, making plastic toy cars and mop buckets, but this is what propelled it to success and fortune. Unfortunately, the image has been difficult to shake.
Taiwan's exports are not as high profile or as well known as other Asian countries' exports. For example, other Asian countries manufacture very well known brands such as Hyundai, (Korea) LG (Korea), Sony (Japan) and Nissan (Japan) and these products can be found both in the home and on the road in western countries, and have had time to become established as brand and household names. Even terrorists roam around aimlessly firing AK47's (USSR) on Toyota pickups (Japan) and are often reported live in HD on your Samsung (Korea) by the news channel RT (Russia).
And let's face it. Many people back home, and some I have met in Taiwan, think that Acer (Taiwan) and HTC (Taiwan) are from Japan (Japan).