This is because until the employee is vested, that employee is under a substantial risk of forfeiture and under Section 83(a) and accompanying regulations 1.83-1 and as such is not subject to current inclusion into that employee's gross income.
This is because until an employee becomes vested, they are subject to substantial risk of losing their benefits and are not included in their gross income under Section 83(a) and accompanying regulations 1.83-1.