It's simple physics; hot air rises. That's why the raised floor has always been seen as the quintesential data center design for HVAC. Hot/cold aisle don't have anything to do with the concept of a raised floor and is only standard for companies that can't afford to keep throwing money at their data center infrastructure. Larger corporations sometimes ignore the hot/cold aisle concept because they can invest more dollars into just adding more HVAC capacity. Smaller data centers where this expense isn't available turn to concepts such as hot/cold aisles, blanking panels, and air stop technology. Though it's all good practice to do all of those things in any event for any data center.
The main reason why companies opt out of raised floor is cost and front-end convenience. Raised floor tend to be more efficient with cool air distribution, more asethetic, and functional for being able to isolate electrical conduits and piping.
In a raised floor the CRAC units are on floor level and are called "down flow" units which blow the air under the floor through an opening on which they sit. Larger data centers will have an airhandler that will plenum the cool air under the raised floor if down flow CRAC units aren't being used.
The primary challenge to a raised floor environment is airflow obstruction because of underfloor infrastructure such as cable trays, conduits, cabling, and piping. That's why a smart underfloor design and execution is critical. You wouldn't run cables and cable tray down an area that would impede sufficient airflow to critical systems. Most data centers get a poor rating for this as they don't actively practice being pro-active and smart in their underfloor design. Newer data centers will combine the concept of raised floor for cooling and overhead trays for cabling.
I believe the standard for the height of a raised floor is 36" minimum though this seems to keep changing over the years. But you need sufficient height to be able to squeeze an average sized human under it.
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