There are a few changes to the amida which reflect the changes in the seasons -- we add (or change to) mashiv haru'ach umorid hagashem (or geshem) at Shmini Atzeret (and back at Pesach) and switch TO v'tein bracha on Pesach.
These changes indicate an awareness in our tefillot of the need for certain changes in nature, and the v'tein bracha is a request: we don't want rain when it would be inappropriate. This is based on our farming needs.
However, Pesach ranges in our calendar from year to year (March 25th to approximately April 24) while the needs of the soil and the sowing and reaping cycle stay the same. In a "late" year, we would be praying for rain for almost a month more than we might need it (or in an early year, for a month less).
Why would our date for changing from v'tein tal umatar livracha be based in a lunar based holiday which moves back and forth through two solar months instead of on a solar date which reflects the yearly agricultural cycle more precisely?