Actually there's really no obvious answer. That's the beauty of history; it is subject to multiple interpretations.
First off, semantically, we have to define what it means to be "developing" or "progressing." It should be noted that certain scholars (especially Petrach, who is mentioned by name in this Wikipedia article) referred to the medieval period (roughly the 5th Century to the 15th Century) as "surrounded by darkness and dense gloom." Petrach felt that the era was marked by darkness: the so-called "Dark Ages." However, that's certainly not a matter of fact. Rather, it's a matter of historical interpretation; a question of historiography.
To offer a competing interpretation, Petrach wrote these comments in the early 14th Century, near the end but still solidly within the period that most historians now consider the Middle Ages. Surely a man living in the time period he is criticizing as being the "Dark Ages" is not unlike what Owen Wilson experienced in Midnight in Paris: the yearning or longing for a time thought to be more exhilarating, more artful, more
plein de vie simply because one was not part of it.
Second, even taking Petrach's comment at face value, his characterization is
at best extreme and at worst somewhat ignorant. The period was marked by significant technological, artistic, religious and economic advancements. For example, accurate mechanical time pieces (time pieces using an escapement mechanism--a technology that is still employed today in high end mechanical watches) were devised during this time period. The printing press was famously invented by Gutenberg in the mid-15th Century. Metal working was highly sophisticated during this period producing a variety of custom-made fully articulated armoured suits for both practical (military) purposes as well as for parades/ceremony. Not to mention the variety of weaponry including crossbows, swords, siege devices (trebuchet, catapaults) and cannons. See here and here.
But to answer your question: "what kept the people of Europe from advancing," a variety of factors influenced Petrach's characterization which has wound up as somewhat of a colloquialism. First, Europe was riddled (or "plagued," if you'll excuse the pun) with disease. The Black Death was estimated to kill somewhere between 30% to over 50% of the population of the European continent. Second, the theory that we as a people identify first with a nation or a country is a somewhat recent development in human history. In the Middle Ages people identified and pledged their allegiance to a "lord" or "master," a concept which formed the basis of feudalism. This sort of "governmental" power structure was not really conducive to getting a lot of things done at a macroeconomic or political level. "Lords" were highly concerned with consolidating land and power resulting in constant infighting, raids, sieges and cutting other people's heads off. See here.
Just that's a quick answer; by NO MEANS intended to be comprehensive. There's much more research to be done.