Does anyone know what the reason was for the change from "Sundays after" to "Sundays of"?

Yes, as it happens. The point about "ordinary time" was to desacralize the liturgical calendar insofar as possible.

In the calendar as it existed pre-Bugnini, it may be said that the events of Salvation History—embodied in the cycle of great feasts commemorating Our Lord's earthly life—persist, in the form of weekly reminders of the feast mentioned after the word after, all through the "fallow" periods after Epiphany and Pentecost. That is to say, for those who use the old calendar, no time of the year is ever ordinary: when a great feast is not being celebrated, its memory and sanctifying effect are being recalled.

The greatest desacralization was reserved for Easter. The new calendar both radically diminishes the impact of the fourth greatest feast day, the Ascension, and waters down the centrality of the events of Holy Week and Easter morning by expressing (hence diffusing) their primary relation to each other and to the remainder of the calendar as one of seasons pure and simple rather than seasons that take their cue and identity from great central occasions. Bugnini's change in the post-Easter calendar terminology also collapsed the significant parallel of the forty-day Lenten period of penitence with the forty-day Easter period of rejoicing in the presence on earth of the Risen Christ.

To give credit where it is due, the new calendar embodies a diabolic brilliance, not least in that its unexpressed revolutionary intent has been very plausibly masked by its deviser's direct appeal to simplification (i.e., as an aid to the faithful) as the actual motive for the changes.