Both protests were anti-government in nature held by civilian populations of the states/countries that were controlled entirely that the single party. The ROC was also in the UN during the 228 Incident, and the PRC was then in the UN during the Tiananmen square massacre.
But why is the attention between the two events so disproportionate?
228 happened in 1947. Tian'anmen happened in 1989. That's an incredibly long span when you considering broadcast technologies and the global news infrastructure. We have and had at the time lots of filmed footage from Tian'an Men but not from 228. That's a big part of it. There were more people documenting the latter.
The other thing that you have to take into account is the situation in China in the 80s. In 1978 the Reform and Opening period began, which was to be a widespread set of reforms, both economic and political, that would eventually propel China into the economic position it's in today. This happened just a couple short years after the Cultural Revolution. When in 1989 there were large scale protests throughout the country which people were seeing as cries for democracy, the Western World was therefore watching very closely to see what would happen.
And they saw what was happening as it was happening. 228 couldn't really even be discussed until the mid 1990s, and most people outside of Taiwan hadn't really had any idea that it has happened at all. You had people like Lynn Miles, but otherwise it wasn't (still isn't) something most people without a connection to Taiwan had any idea about.
tl;dr: The two events happened under very different circumstances in very different global political environments and with very different levels of availability of news in the West as events were unfolding.