Permanent Enchantment is a lost art
Thanks to scientific advancements, the sorcerers today are much more powerful than they were 1000 years ago, but somewhere along the way we forgot how to make enchantments permanent. So, while a spell can solve any immediate problem, it can't really be used to solve a continuous problem very well. For this we use technology. But the ancients could cast permanent enchantments on things and thus solve continuous problems without the use of technology. How they did this, we do not know, but we now call it magictech.
We see similar things in our own real world history. For years, ancient technologies like Damascus steel and Roman Concrete baffled modern science. When we finally figured them out, it turned out the secret was in the impurities that these ancient civilizations did not remove from their works as opposed to something they intentionally put there.
In Rome for example, they would mix their concrete with whatever water was most available (typically sea water) instead of fresh drinkable water like we normally do today. The salts became part of the concrete's structure and made it far more resistant to salt water corrosion.
In India, there is a common impurity in iron ore that you don't find in other parts of the world that was responsible for forming the higher quality Damascus steel.
In ancient times, this made steel from India especially sought after, but during the industrial revolution, improved refining practices began removing this impurity from the ore making it fundamentally no different than any other iron ore. So when scholars began comparing modern steel with Damascus, they were shocked to learn how good this ancient steel was, and for decades, they were not able to figure out how they did it; so, for decades we called it a lost technology.
These technologies were difficult for modern science to solve because we kept trying to figure out what they were adding to their materials when in fact, the secret lied in what they were failing to remove. So even when we studied their writings we could not figure out the secret, because even they did not know what made their materials better.
Likewise, during your setting's dark age, sorcery could have continued to evolve. While it may have regressed in some areas, it would have also moved forward in others. So, imagine magic comes from some ethereal source that surrounds us. To case a spell, you must extract mana from the ether, and then manipulate it into its desired form. But the ether is not just a cloud of mana, it contains all sorts of energies, many of which are detrimental to casting; so, the first thing any sorcerer from the late dark ages onward does is isolates the mana from other ethereal energies allowing him to maximize the power and predictability of his spell. It is so ubiquitously done that it is the very first thing young mages are taught to do, but in the ancient times, people did not isolate mana very well to cast spells; so, one group of wizards found that when they cast spells on certain lay lines, that they could be used to make permanent enchantments.
The reason these lay lines don't work for modern mages is because it does not occur to them to use the unpurified mana that exists there; thus, this ancient form of spell craft is lost to your modern civilization, and with it the kind of magic they need to make Magitech viable.