What was the main reason for the entry of the majority of Russian principalities into the new large states?

This reason was that Russia was fragmented. The principalities were part of the stronger states. On the one hand, it was an attempt to avoid the Horde yoke, and on the other hand, it was impossible to resist an external invader. So, for example, at that time the Grand Duchy of Lithuania arose. A significant part of the southwestern lands of Russia was included there. The Russian population here was on an equal footing with the Lithuanians.