RLM explained that it wouldn't make sense for Anakin to construct Threepio to help Shmi by observing that a protocol droid wouldn't be much use in the home. If Lucas has to have Anakin build one of the two droids, it is more logical for him to build Artoo (to help himself) and have Threepio belong to the Queen, whom he can help with negotiations. While Artoo certainly proved useful on the Queen's ship, a ruler might reasonably be expected to have more use for an interpreter than a mechanical droid on a day-to-day basis.