With the series connection, the pump will have less that 12volts across it and is unlikely to run, similarly the solenoud valve as just over 12v and is unlikely to pull in.
In the parallel case, both the valve and pump will have 24v when the PSH and PSL are closed.
I suppose you could say that God set up the rules so that parallel and series electrical circuits operate in a logical manner that can be predicted mathematically. As to why, you'll have to ask him/her when you get the chance.
It would certainly have been a much different universe if it had been designed without being mathematically coherent, don't you think.