The employee is likely to underperform for the first few comp cycles after being hired, due to the new role, new context, etc. So the employer is willing to guarantee an "optimistic" salary up front for the first year (or, with equity vests, perhaps longer).

But the employer is unwilling to guarantee this indefinitely; after a year or two, they want to see you performing at level--and thus qualifying for merit increases--or else you'll revert to the lower salary.

Viewed this way, a rational employer will offer you a lower total comp for the first year if it's all salary; taking more of your first year's comp as bonus represents a bet on your own impact (and the fairness of the merit-based comp modeling).