Why is Tolarian Kraken worded this way?

@murgatroid99's answer is exactly correct, in my opinion, but I wanted to go deeper on illustrating his point.

Consider the two cards Savai Thundermane and Lightning Rift: both have an ability that can somehow deal two damage to a creature when a card is cycled. Now consider what happens when your opponent has these cards and you have a 2/2 creature you want to keep alive as well as a Ranger's Guile.

In the case where your opponent controls Savai Thundermane, they will need to actually pay the two mana in order to cause the second ability to trigger, at which point they will target your 2/2 and you can respond with the Guile to save it.

In the case where your opponent controls Lightning Rift, the burden is on you to act first. If you want to save your creature, you must cast the Guile immediately and your opponent will not have to waste their mana. If you wait until you know whether or not your opponent actually is willing to commit the mana, you no longer have any ability to respond and stop the damage.

Anecdotally, this situation where something may or may not happen but the opponent must act as though it will can certainly cause confusion. I remember a tournament game long ago where I controlled a Cursed Scroll, my opponent controlled a Ravenous Baloth. My opponent was at two life and I held five cards. I activated the Scroll targeting him, he let it resolve, I wound up revealing the named card, and then he was baffled at why he could not respond by sacrificing his Baloth. It seems that modern-day cards are trying to avoid these situations.

Imagine being faced with the decision to sacrifice or activate a creature simply because another player started their turn? By using a delayed triggered ability, the need to make that decision is delayed until after the cost is payed.

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"When you do, ..." is triggered ability.

603.1. Triggered abilities have a trigger condition and an effect. They are written as “[When/Whenever/At] [trigger condition or event], [effect]. [Instructions (if any).]”

Specifically, it's a variation of delayed triggered ability called a reflexive triggered ability.

603.12. A resolving spell or ability may allow or instruct a player to take an action and create a triggered ability that triggers “when [a player] [does or doesn’t]” take that action or “when [something happens] this way.” These reflexive triggered abilities follow the rules for delayed triggered abilities (see rule 603.7), except that they’re checked immediately after being created and trigger based on whether the trigger event or events occurred earlier during the resolution of the spell or ability that created them.

The ability is only created on resolution.

603.7a Delayed triggered abilities are created during the resolution of spells or abilities, as the result of a replacement effect being applied, or as a result of a static ability that allows a player to take an action. A delayed triggered ability won’t trigger until it has actually been created, even if its trigger event occurred just beforehand. Other events that happen earlier may make the trigger event impossible.

This has two implications.

First, the target is chosen when the delayed triggered ability is placed on the stack, not when Tolarian Kraken's ability is placed on the stack. This is inconsequential.

The other, relevant implication is that this gives a chance to the players to react to the delayed triggered ability. This is beneficial to the opponents of Kraken's controller since they know the cost has been paid before having to react.

Doing otherwise would make Tolarian Kraken an overly powerful card. Imagine being faced with the decision to sacrifice or activate a creature simply because another player started their turn? With the current wording, Tolarian Kraken's controller must commit to paying for the ability before forcing a response.

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[This is effectively the same answer as murgatroid99's, but it explains why the targets are chosen later than they would be with if.]

Waiting until after the cost is paid to choose the target is the big difference here.

If the ability used "if you do", like on Throwing Knife, then an opponent who wanted to "save" their creature from the ability would have to do so before they even know if you are going to pay for it. That gives the ability the unintentional power of forcing your opponent to react to the possibility of paying the cost, without actually paying any cost. With the "when you do" wording, the opponent can react to your choice to pay the cost, making it more like an activated ability.