The German colonists were invited for a reason; they brought with them more advanced agricultural practices, manufacturing techniques and other useful forms of knowledge which contributed to faster urbanisation in Bohemia/Moravia/Silesia.
The same was introduced to Poland (also to territories where Germans colonists didn't come or came in insignificant numbers). By agricultural practices you mean mainly the 3-field-rotation system. This appeared in Poland already in mid-12th century. By late 13th century was already widely spread.
When we speak about development of Poland in 13th - 15th centuries then one should also not forget that not every part of Poland was equally developed. Generally most developed parts of Poland were Silesia (later during this period it became part of Bohemia), Land of Cracow, Greater Poland and Cuiavia - level of development of these was roughly equal (but Greater Poland, Cuiavia and Land of Cracow managed to catch up with Silesia only in late 13th / 14th century). Land of Sandomir followed closely. Mazovia was visibly behind southern & western Poland even though it was densely populated (but it was mostly rural, urbanization level was much smaller than in Greater Poland / Lesser Poland). Pomerelia (with Gdansk) was also one of less developed regions.
Population density wasn't a cause, it was a consequence - a measure of how well-off the land really was.
Not really since I already wrote that population density was bigger in Bohemia yet in 11th century (before any German came there).
Also large population density was not always a real measure of how well-off the land (and especially its people) really was / were.
Overpopulated areas in Medieval had very significant number of people living in poverty - this also refers to large towns.
In Poland Mazovia was even slightly more densely populated than Lesser / Greater Poland, while being backward compared to them.
In 16th / 1st half of 17th century Poland was not less well-off than most of Western Europe, while still being less populated.
But Poland's wealth in 16th century was based mainly on its export of agricultural goods to overpopulated West.
You've always been a backwater
Actually no. From the mid-14th century to the late 16th / mid-17th century one can observe a fast development of "industrial sector" (as we would say today) of Polish economy as well as agriculture and towns (urbanization) as well as education (literacy level, for example). Of course I am talking here mainly about Greater Poland, Lesser Poland, Royal Prussia (Pomerelia) and Mazovia - core of Poland. Not about the PL-LT Commonwealth as a whole.
What destroyed it, impeded the Polish social-economic development and doomed Poland for being a backwater for the next 300+ years were the destructions caused by wars which started in mid-17th century. Especially the so called Swedish Deluge was devastating for Polish economy because while Khmelnytsky's Uprising, wars with Russia na such devastated already mostly underdeveloped parts of the country (i.e. eastern territories of the Crown of Poland and territory of the Grand Duchy of Lithuania), the Swedish Deluge devastated this part of the Commonwealth which produced most of its GDP.
the relatively small size of the Bohemian lands coupled with their proximity to the other Imperial lands allowed them to adapt and absorb Western trends faster and to trade with them more. Poland and Hungary were much bigger and more isolated by their geographical position.
Poland ceased to couple with the proximity of its lands only after the Union of Lublin of 1569. This dispersed the economic and demographic potential of Poland into vast areas of Ukraine and Lithuania. As the result it hampered and slowed down further socio-economic and demographic development of actual Poland. Eastern expansion is what doomed us. The "Kresy Empire" Poland built was too vast for it to efficiently maintain.
Not mentioning the fact that Union with Lithuania & accession of Ukraine added some new strong enemies - mainly Moscow (Russia) and Ottoman Empire. So also military potential of Poland was dispersed as Poland had to help GDL to defend its eastern borders and had to defend Ukraine.
That's why I wrote yet several pages ago that Poland did not benefit at all from its Union with Lithuania in long term.
