Because he has always been so open about his family in the past....

Remember that multiple shifts in time and dimensions also happened so we really cannot be entirely sure if Spock is related even to himself.

As that was one of my big criticisms of Discovery I appreciate the insight.

However, it still felt like a giant lazy shoe horn mandated by some out of touch exec in order to bring some fan service to a new show.

So it's apparently Star Trek tradition to introduce new characters and reveal they are another, previously undisclosed, member of Spock's family.

Some future series is going to have a really wild episode where a character just casually reveals they are Spock's child.