Well, the most basic thing is: renewables are completely helpless alone, they need actual, real power generation (baseline, which are not dispatchable). If you let negative prices run too long (coupled with other incentives that skew the market), the baseline companies might go bankrupt and you won't have power when renewables decide not to work. Negative prices also more or less mean that your grid is overstressed and in risk of damage. Those are just the two most obvious (to me!) reasons.