Admittedly my involvement with motorbike engines has been largely with MotoGP power units where electrics and tyre development is way ahead (and necessary) of speedway units although the basic engineering concept still applies to both, namely, torque is required to get the bike moving and revs (horse power) needed to maintain the delivery of power.
Speedway engine builders will be aware of this and will always be faced with juggling to balance these two factors which are not mutually exclusive to one another & hence engines can be built/adjusted to varying degrees of torque & HP for different types of track/circuit
The lower center of gravity of the lay down machines v the uprights would them easier to lean over and create an easier break of the tyre/surface and thus turn quickly if and when required although most probably no easier to race on over four laps - just different.