I cannot tell you "why," but would think that this would be a good Feature Request for Adobe. Tell them how useful the Favorites was to you, and request that they be reinstated.
With but a few exceptions, I use only Butt-Cuts, Dip-to-Black and Cross-Dissolves, and almost none of the rest. Usually when I do need another, I build it myself from Pixelan's Spice Master. For me, it's easy to just grab the one that I need, when I need it. I never do any Transitions in a random pattern. Each is throughout carefully, and connotes a particular "device," usually to express the passage of time, or a change in place. However, that does not mean that others would not find the Favorites extremely useful, and if there are enough, who do, then Adobe should reconsider.