Script evolution changes in time, and here are some of the reasons:
1- Geography, language, existing culture and cultural influences from the outside and finally, history (whether war, annexation, colonization, etc).
trajan, circa 1st-3rd centuries ad (trajan column)
2- Trajan, above, becomes Rustica, below,
rustica, circa 5th century ad
Why? Nobody really knows. Why are Latin peoples more boisterous than the Nordic? Climate? Food? Language? ALL OF THE ABOVE. I made the point to compare Romanic vs. Gothic. The form in the south is more rounded and expanding, the north is ascetic and ascending.
3- With the advent of papyri and vellum we get: NEW ROMAN CURSIVE (miniscules). THE INK FLOWS! It was used from approximately the 3rd-7th century, and uses letter forms that are more recognizable to modern readers; "a", "b", "d", and "e" have taken a more familiar shape, and the other letters are proportionate to each other rather than varying wildly in size and placement on a line.
roman cursive (see that this is a faster trace, like gel ink in paper today)
4- We also get UNCIALS (majuscules), this is the lettering of power, church, aristocracy, royalty. Uncials spread all over: west, east, north. Greeks loved uncials! Byzantium loved uncials! Why? I guess it's haiographic.
This is around 7th-9th centuries Carolingian in the west, Byzantium in the east.
uncials 7-9th centuries
5- Then we get half-uncials. What do they do? They allow for serifs! That's the beginning of ascenders and descenders (remember half-uncials have nothing to do with uncials, all it means is that it's a church letter! Hymns, Psalters, etc).
6- Now the half-uncial crosses the channel to the British Isles. Where? Ireland! Why? The answer may be in the Book of Kells! The Irish were really into illuminated texts... (and they mixed it with keltic paganism).
or this (much later) 11th century, miniscule:
Merovingian, basically a Carolingian miniscule circa (7th-8th century)
and Beneventan (basically derivation of Carolingian miniscule),
In time you get this beauty. The Gothic Textura!
Mature Gothic is already the Humanist style.
Bastarda, Chancery, secretary, etc, and the rest are just variations of these styles.