I think it's because most maps it would be unsafe and way too risky to go for the relics right away. In arena the relics are easy to scout and you have good walls so it makes sense to go for them asap. Also monks are used more in general with arena since once again you are safe enough in the walls that it's not a huge risk to make them.
You tend to see relics play more of a role in 1v1 black forest as well for similar reasons, you always fc and it tends to go late game where the extra gold really makes a difference.
This + monks being considerably more OP on arena than other maps because of the space factor, which as said effects relic distribution aswell. So these 2 element feed each other into a chaotic battle for relics! :twisted:another reason is that since there is so much wood around the boarders of the map the relics usually are pretty close to each other and easy to scout + to gather ...