People believe they are getting a bargain.
Ficus is the cheapest because it is extremely easy to propogate and inexpensive to grow to a sellable size, e.g. low fertilizer cost. Ficus is the fastest growing hedge material. Nurseries are, therefore, able to turn over inventory and make more money than growing and selling other varieties of hedge materials. Ficus is also extremely fast growing in the landscape. Glad you asked Ficus bejamina is a actually a tree . .