With the collective land, land is owned by the commoners working it, with the monarchs above being able to exert less control over them. The commoners, on the other hand, are more able to invest their wealth into improving their status, resulting in higher development growth. This is not a state owning land and the people working it, this is the concept of the people as a collective owning the land they work - but as a monarch, this means you have less complete ownership of that land (and the people).