I'm not sure that would work. The current format makes it an effect that happens "as x enters the battlefield." The proposed wording is intuitive as to when you pay the 2 life, but it isn't explicitly stated, which might make it not work from a rules perspective. I'm not on the rules team, though.
There is no problem with either wording ruleswise. A replacement effect for how a permanent enters the battlefield doesn't have to use the phrase "as ~ enters", there are other possibilities. Here are the two rules that each cover one of the two wordings:
614.1c Effects that read “[This permanent] enters the battlefield with . . . ,” “As [this permanent] enters the battlefield . . . ,” or “[This permanent] enters the battlefield as . . . “ are replacement effects.
614.1d Continuous effects that read “[This permanent] enters the battlefield . . .” or “[Objects] enter the battlefield . . .” are replacement effects.
As Breeding Pool enters the battlefield, you may pay 2 life. If you don't Breeding Pool enters the battlefield tapped.
Breeding Pool enters the battlefield tapped unless you pay 2 life.
There is no problem with either wording ruleswise. A replacement effect for how a permanent enters the battlefield doesn't have to use the phrase "as ~ enters", there are other possibilities. Here are the two rules that each cover one of the two wordings:
614.1c Effects that read “[This permanent] enters the battlefield with . . . ,” “As [this permanent] enters the battlefield . . . ,” or “[This permanent] enters the battlefield as . . . “ are replacement effects.
614.1d Continuous effects that read “[This permanent] enters the battlefield . . .” or “[Objects] enter the battlefield . . .” are replacement effects.
As Breeding Pool enters the battlefield, you may pay 2 life. If you don't Breeding Pool enters the battlefield tapped.
Breeding Pool enters the battlefield tapped unless you pay 2 life.
As a further example, Clone's ability before Magic 2010 once read "As Clone comes into play, you may choose a creature in play. If you do, Clone comes into play as a copy of that creature" (compare the Tenth Edition card with the Oracle text [C.R. 108.1]) Since Magic 2010, the text uses a shortened form (as well as the new term "battlefield" [C.R. 403.5]) which doesn't change how Clone works.
In general, however, questions that ask why one wording is used on a card rather than another (especially where one wording is alleged to be better) are questions that only Magic R&D can answer.
Could the shocklands have said...
Breeding Pool
Land – Forest Island
Breeding Pool enters the battlefield tapped unless you pay 2 life.
...?
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)
Yes, and we are probably going back to Ravnica soon enough so we could see them worded this way.
Thanks for blazingly quick answer
As Breeding Pool enters the battlefield, you may pay 2 life. If you don't Breeding Pool enters the battlefield tapped.
Breeding Pool enters the battlefield tapped unless you pay 2 life.
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)
In general, however, questions that ask why one wording is used on a card rather than another (especially where one wording is alleged to be better) are questions that only Magic R&D can answer.