Excellent question! The book is right, but ... well, biology has a hard time with the word because. We read causes and effects into evolutionary designs to make them easier to remember, much as the ancients linked everything to the four humours. There is an answer to because, somewhere in a very long period of geological history, but take all assertions (even mine!) with a grain of salt. I will, of course, use this word quite often below, with great and misplaced confidence.
I would disagree with the statement that we don't need a renal portal system. There are unfortunately many cases of acute kidney injury that result from something as straightforward as blood loss. The kidney needs to shut down the blood flowing in (constrict the afferent arterioles) because filtrate production would further reduce the volume of the blood. However, the kidney itself depends on the blood coming out (efferent arteriole; peritubular capillaries; vasa recta) to sustain its own needs. It often dies in the attempt, and without kidney dialysis or a lucky transplant, so would the person. The renal portal system would have prevented that by allowing blood to reach the peritubular capillaries (and presumably the vasa recta once it evolved) by non-leaky vessels. The portal system means that blood is at less-than-arterial pressure, comparable to blood after it has been put through the glomeruli. The system is cunningly wrought so that valves in the veins can force blood into the portal system only when the normal arterial input is reduced: (Akester, 1967)
How did we lose the system? I have no idea. Sometimes we lose things like the enzyme to make vitamin C apparently by pure chance - at some point in time, our ancestors didn't need it for a while, and key genetic information was corrupted because natural selection didn't fix it. Other times, there is a small overhead in metabolic cost or the potential for other disease. Maybe the excellent adaptations of mammals for water recovery with a long loop of Henle were incompatible with the structure.
What is clear is that we can do without it, normally, unlike many other species that frequently shut down some or all of their glomeruli by constricting afferent arterioles. This is related to the heart, since the blood goes directly from aorta to renal artery instead of passing through the capillaries of the lung first, and therefore has a consistent high pressure (Keogh, 2021). But it is also related to the issue of why we aren't shutting down urine production. Mammals have a consistently high metabolism, requiring constant internal heating. This goes beyond mere sugar consumption - our active bodies are continually generating urea, uric acid, creatinine, and other wastes that contain nitrogen that need to be flushed out of the blood. So we pretty much do want to keep the glomeruli going all the time, and we have a powerful four-chambered heart that does that effectively while powering the rest of our lifestyle. By contrast, reptiles are known for sudden bursts of action and long periods of inactivity. Their cold-blooded physiology has much less need for food, and I think they don't need to excrete waste so consistently.
I should apologize for not keeping the answer simple, but understand this is a question that would stump good college anatomy and physiology students! I've started linking some terms above - feel free to ask for clarifications on anything.