It's not just the cost of the hi/lo, they'd also need to buy dollies to transport the ULDs on. These take up a fair bit of space when stored too, which it's fair to say is at a premium at SOU!
The lack of this equipment doesn't stop the airport handling fuel & go diversions, bulk loaded aircraft, or containerised diversions on the understanding that the bags stay onboard, so personally I don't think it's a huge issue considering the cost/benefit ratio.