Apart from reasons highlighted above, turning trains at Huyton as well as having through trains is slightly impractical.
You’d have to turn the TPE back in Platform 2, which is also used by Wigan-Liverpool services, twice an hour. Doable, yes, if the train is in and out but what pathing along the Chat Moss and then at Victoria, if you running it at a different time?
So instead you’d have to shunt the unit behind the signal at Huyton Junction and into Platform 4, again working around 2tph to Wigan, any late running and you’re asking for trouble.
Not only that, you have to think of traincrew breaks, you’d have to taxi traincrew Huyton to Lime Street, which is booked to take 30 mins. You can’t have PNB’s at Huyton and you can’t leave units mainline at Huyton with no traincrew, you have to a member of traincrew with the unit at all times, even with the units fully berthed/disposed, again think of traincrew relief issues/PNB’s related to this.
All of those sound eminently resolvable to me