For the same reason the world didn't take climate change seriously already in the 90s. It didn't align with short-term economic interests. As mentioned in the article, the prevailing political climate had made energy imports less political and more a matter of economics. Increasing energy imports from Russia made sense in that climate.
The Russian invasion of Georgia wasn't a big deal in West Europe. It was like the US invasion of Iraq. An empire invaded a faraway country because its leaders had some personal issues. That's what empires do. Political leaders condemned it, but there were limited practical consequences, because everything happened far away. And because the EU was not trying to be an empire shaping the world in its image.
East European countries took things more seriously, because they are closer to Russia and more vulnerable to its whims.
Even the 2014 invasion of Ukraine wasn't a major issue. Until then, Ukraine was seen as one of the corrupt post-Soviet states that didn't know whether they wanted to align with Russia or Europe. As long as Putin's ambitions were limited, they were not a West European concern. Leaders again condemned Putin's actions and some sanctions were issued, but that's it. Putin's Russia was still seen as a fact of life West Europe had to learn to coexist with.
Things changed after 2014. Ukraine aligned with the West. When Putin invaded the next time, it was a full-scale invasion of an unambigously European country. That did concern West Europe, because it was dangerously close.
The whole situation looks like a mutual misunderstanding of the motives on the other side. Putin seems to have counted for a limited European reaction, because he didn't understand that non-empires tend to make pragmatic rather than ideologically motivated decisions. He assumed that West European complacency was an ideological rather than a pragmatic position. Similarly, West European leaders didn't see that the threat of the invasion was real, because they assumed empires would also make pragmatic rather than ideological decisions.