Why would successive dynasties "strain a point"?

Directions: Read the following passage and answer the question that follows:

The year 842 BC may be considered the first accurate date in Chinese history, and in this year the Emperor had to flee from his capital on account of popular dissatisfaction with his tyrannical ways: he betook himself northward to an outlying settlement on the Tartar frontier, and the charge of imperial affairs was taken over by a regency or duumvirate.

At this time, the confederation of cultured princes called China — or, to use their own term, the Central Kingdom — was a very different region from the huge mass of territory familiar to us under those names at the present day. It is hardly an exaggeration to say that civilised China, even at that comparatively advanced period, consisted of little more than the modern province of Ho Nan. All outside this flat and comparatively riverless region inhabited by the "orthodox" was more or less barbaric, and such civilisation as it possessed was entirely the work of Chinese colonists, adventurers, or grantees of fiefs in partibus infidelium (so to speak). Into matters of still earlier ancient history, we may enter more deeply in another chapter, but for the present, we simply take China as it was when definite chronology begins.

The third of the great dynasties which had ruled over this limited China had, in 842 BC, already been on the imperial throne for practically three hundred years, and, following the custom of its predecessors, it had parcelled out all the land under its sway to vassal princes who were, subject to the general imperial law and custom, or ritual, together with the homage and tribute duty prescribed there under, all practically absolute in their own domains. Roughly speaking, those smaller fiefs may be said to have corresponded in size with the walled-city and surrounding district of our own times, so well known under the name of Hien. About a dozen of the larger fiefs had been originally granted to the blood relations of the dynastic founder in or after 1122 BC; but not exclusively so, for it seems to have been a point of honour, or of religious scruple, not to "cut off the sacrifices" from ruined or disgraced reigning families, unless the attendant circumstances were very gross; and so it came to pass that successive dynasties would strain a point in order to keep up the spiritual memory of decayed or rival houses.