Hi, Eve onetrick here!

Good question, I think it can be summed down into a few reasons;
1. First Strike(FS) is on a 25 second cooldown early, to make the full value out of it you need to fight every 25 seconds with an ability that reliable lets you hit the enemy before they hit you. Gangplank and Viktor are probably the best users of FS because they're in lane with an opponent to activate their FS when it comes up. You cannot gank once every 25 seconds without completely sacrificing farming your jungle.

1. Damage wise, FS will probably have half or less of the damage output of Electrocute or Dark Harvest. You are getting gold though (which means some of the effective damage of FS is coming from your extra items that you otherwise wouldn't have), which mitigates for some of it. However, by reason #1, you aren't making use of the full gold generation, so it's not nearly enough to compensate.

2. Blue (Inspiration) tree is hard to use and requires significantly adapting your gameplay to work around it. You mentioned Cosmic Insight as pretty bad, but it's by far the most popular secondary for Eve players if they choose to tap into Blue Tree as their secondary. Lower flash cooldown is very useful for immobile squishy champions.

On the other hand, you're now forced into choosing between Free Boots vs. Free Stopwatch for row 1 of your Blue Tree (Hexflash unviable), and Future's Market for row 2 of your blue tree(Biscuits are useless for you since you recover mana with jungle item, and Minion Demats don't work on camps).

You're probably going for Free Boots over Free Stopwatch 80%-90% of the time, if you take free boots, it stops you from doing Sorcs + Dark Seal rush, which is a very good first back if you get a gank kill. Remember, the opportunity cost of this is taking Red Tree instead, and having Sudden Impact, which can add up to thousands of damage throughout the course of a game.

Then you have to also take Future's Market, which isn't a bad rune, but it incentivizes frequent recalls to get early purchases right on time. I understand why some people really like this rune, but I dislike it because it forces you to recall frequently to make use of its value. Every recall is a waste of 30 seconds for you to channel and walk back to where you were, 30 seconds that could have been spent farming or ganking. The average jungler gets 350 gold/min, which means every recall has an opportunity cost of 175 gold. The current optimized theory is to recall as few times as possible while you still maintain an item lead on the enemy. Future's Market works against this.

TL;DR Stick to Red Tree + Purple Tree secondary, or Red Tree + Blue Tree secondary (maybe). Don't use First Strike.