Different laws. Congress Critter (House) vs. Senator. Senator vacancies are appointed, congress critters get a new election (and we'll have two extra elections - one to elect someone to hold her seat until the result of the "special election" plus the "special election").
The rats aren't saying much about it costing the state $4 million.. ($2 million per)
It doesn't matter how much it costs as long as it benefits them but if it had benefitted a Republican then of course, its just a waste of hard earned american tax dollars.