Sorry to be slow getting here, filling orders is keeping me nose to the grindstone.....
With respect to the authors of "I See By Your Outfit", but that time line is quite a few years off......
For what it's worth here's something of a partial history of chaps, which is based on my research of 35+ years: studying the literature (saddle shop catalogs are an excellent resoruce), period photos, and examining hundreds of original chaps in museums and private collections (unfortunately for the serious student many of these pieces never see/will never see the light of day so to speak and many offer styles/construction, etc. not seen elsewhere.......)
Chaps (from the Spanish chaparejos and/or chappareras) are descended from the Mexican vaqueros armas which were skirt like pieces of bull hide tied to the saddle horn to hang down in front of the legs which developed into armitas which were smaller, generally belted around the waist, and tied around the legs with thongs. Add the influence of Indian leggings, two step in tubes of leather with no seat and often with fringe along the outer seam, and you have the earliest form of chaps, the so-called shotguns.
Shotgun chaps were originally developed during the late 1860's in the Texas brasada (brush) country where pretty much everything has thorns, thus they were made to cover as much of ones lower section as possible, including the crotch area. The earliest chaps had a one piece belt (infrequently used in later times, on page 68 of "Cowboys & the Trappings of the Old West" there is a pair by R. T. Frazier, probably 1890's - Frazier's first shop was in the 1870's in Leadville, Co, and he later moved to Pueblo, Co in the 1880's. He partnered there with S. G. Gallup before opening his own shop in 1898, where/when the pair shown were apparently built) and were mostly plain with no fringe or conchos. The one-piece belt evolved fairly quickly into the two-piece, straight top waistband, which was laced together with usually 5-10 lacings. This style waistband is not only easier to build, but is more cost effective due to time savings and the maker can also use smaller sections of hide which might otherwise have been scrap (scrap is a big issue with leather smiths). This style waistband was used from the 1860's into the early 1900's. This style waistband/crotch is bulky though and can be uncomfortable - more on that later.
The curved top waistband (still with a fairly wide and closely laced waistband) first appears in the late 1880's early 1890's. The book "Cowboy Culture", by M. Friedman, includes a pair of this style by F. A. Meanea, of Cheyenne, Wy, with a date of 1880-90's. Here's a picture of said pair:
Dating such things can of course be difficult/subjective if not accompanied by written documentation/provenance, but in this case I have some pretty good verification. At one time I owned a pair of chaps that were an almost exact twin to the pictured pair. When I got them they were like new, and in the pocket was the original Meanea sales receipt dated 1889 (unfortunately I no longer own them or have a copy of the receipt). This style waistband doesn't appear as often as the straight waist until the late 1890's and after 1900.
As for the less complicated lacing "Cowboys & the Trappings of the Old West", has pictures of several pairs of rodeo chaps with curved waistbands and with less lacing in the 1905-1915 era.
Woolies, were developed in the late 1870's-early1880's, originally for wear in the cold north country. Because they were flashy, this style became popular in the 1890's and into the early 1900's with Wild West and rodeo performers. Usually of Angora goat hair, they were also made of bear hide, seal skin (see those worn by Theodore Roosevelt who BTW hated being called Teddy, he preferred Theodore or TR), and even dog amongst other skins.
I've got to go for now but I will try and add as time allows regarding other changes such as the buckle and strap leg attachments rather than solid/laced legs, fringe and conchos, the change in waistband lacing which begins around 1900 (Del's picture clearly shows this change at least as early as 1905 and not the later date as noted in "I See By Your Outfit"), the beginnings of the batwings (the original style had narrow "wings, buckles and straps, and were called the Cheyenne style - they date to at least 1887), and more.....
Why the changes? comfort and the Wild West Shows/Rodeos were large influence on the changes in all kinds of cowboy gear during the 1880's and later........
as always others mileage will vary and further information is welcome........