The simplest answer to the question is that WikiTree does not accomodate dates before the year 1. Even though the objective of putting together a tree together that includes every living human is itself a virtually impossible dream, virtually no human alive today can trace their ancestry back, with reliable documentation, that far.
Having such a cut off is beneficial to WikiTree because we avoid having to address number of interesting but contentious topics, including,
* the added mechanical wrinkle of addressing dates which get larger as they go backward -- dates which would not have been recognized by the people to whom they refer
*dealing with the anomaly that, given the historical references in Jesus' birth stories, historians identify the date of Jesus' birth as about 4 BC.
* referring to dates as AD/BC ("Anno Domini/the Year of our Lord" and "Before Christ", which are essentially expressions of faith), vs. CE/BCE ("Common Era" and "Before the Common Era" which are today's preferred secular references to dates).
* debating whether Matthew or Luke has the more authoritative ancestor list for Jesus, since they are different.
Fortunately, there are are surely plenty of other forums in which these fascinating discussions can take place!