It is for weight reasons. They do clear blocks slower when empty but the system is usually ok with that. The top section, before the final drop has the boats moved along by underwater tyres rather than a water flow. When the boats are too light. Not only do they struggle to make it to the next set of tyres, they can struggle to get enough traction to actually push themselves across the tyres fullstop, due to the boat not having enough draught.
I've spent many times going up there, rain and shine to help retrieve a stuck boat back in the day. Usually involved 2 of you pulling / pushing the boat from thr catwalk until such a time the boat does regain traction on the tyres near the drop. If you look at them Storm Force 10 pictures posted a few pages back. You can see the tyres and motors in all their glory.
They can also occasionally get stuck in the splash down zone for the reverse drop, before reaching the friction tyres and second turntable. The design for that drop is ever so slightly flawed. It prefers an ever so slightly higher waterlevel, which they can acheive by covering a few of the water release holes in the trough near the screw pump. But that causes problems else where with sensors and other equipment. So it gets ran at a lower level, tonensure maximum ride reliability and pretty much always has, still well within the safety margin though. That only happens occasionally. Never when the boat is full. So aslong as boats are weighted and the system has the slightly lower water level. The ride usually runs flawlessly.
It is a far cry from the simplicity of water rides such as the old Flume at AT. Much simpler rider system. But that also gave it more reliability and the ability to operate boats at any weight with almost no chance of it getting stuck.