... Even more so if you use more than 5 tags.
The first tag you use is obviously the most important one, because it is the primary tag. Once set and published a post with a certain primary tag, you can't change it. While all the other tags you can change (as in remove or add others, which allows you to change their order as well).
Before tribes made their appearance in the Steem ecosystem, there were a maximum of 5 tags one could use for a post.
But with the advent of tribes, and the use of tags as an indicator of including a post in a tribe and in sharing rewards from its pool, 5 tags suddenly became insufficient.
Shortly after tribes were introduced, their interfaces started to allow 10 tags instead of 5. That's more like it, people would say! Then SteemPeak supported 10 tags as well.
But... there was still an interface which was stuck with 5 tags: steemit.com.
So, why that? It was an easy change really.
I believe they didn't make the change immediately, so that people get used to adding their most important tags first. If they cared whether their content is seen on steemit.com for that tag or not.
Sure, the same rule was in place for all interfaces, but it's much harder to notice it, if you have 10 tags at your disposal.
But if you look at SteemPeak, here's what it says:
Have you noticed this explanation before? I'm not sure I have. If I did, it slipped my mind after a while. But I remembered that steemit.com had only 5 tags and I made a habit to add the important tags first, and tribe-related tags, for example, at the end.
Recently steemit.com also increased the number of supported tags to 8. But 
Probably this doesn't help very much those content creators who add 10 tribe tags, but for anyone who also uses topic-relevant tags, knowing and applying this is important.