Well part of it is that the events at Tiananmen Square occurred much more recently than the 228 Incident in Taiwan.
The other big thing is that (I'm largely assuming) you are probably writing from a Western perspective. Taiwanese and Chinese people certainly discuss and know about 228.
The time factor is important because of two things.
The number of people who were at, know people who were at, or in general know about what happened at Tiananmen Square are much greater than those for 228 in Taiwan. They are younger and still very much 'active' for lack of a better word.
Footage of what happened at Tiananmen Square was widely seen across the world and widely reported on in real time. The technologically different times have a big impact on how the two incidents are perceived.
The other factor is that the Taiwanese government has ceased suppressing information and dialogue on the events in the late 1990s. They have since been open in addressing 228 directly, as well as offering public apologies and attempting to make amends with the families of those involved.
The PRC on the other hand still steps around the issue and certainly does not allow open discourse on what happened at Tiananmen Square so it remains a juicier subject, whether on the air, in print media, or on the web. Academia is no safe place for research either. The PRC certainly has not compensated families, admitted to wrong doing, etc. Censorship on Tiananmen Square continues to this day.
Finally, pop media has a massive hand in how people remember the Tiananmen Square protests. There are even songs inspired by the events. Meanwhile 228 has almost zero presence in popular media in the West.