Because the range of rather ill conceived and/or poorly-described mechanics in a lot of the games of the late 1970's and early 1980s.
This was an era where home computers were poor at editing, layout (even professional) usually involved strips of lead, and RPG's were often written by people who would have failed the English and History classes I had to take for my BA in History. And I do mean failed. The typo rate in AD&D 1e was high enough to at best get a C+ from the History professors and a D from the English. Palladium was a typo a page or more. RuneQuest was better, but still problematic. Traveller was probably the best edited and best laid out.
At a very basic level, most post 1985 games are far better organized, far better written/edited, far more consistent, and often, better laid out... as home/small-office computing had good editors with spellchecking. Desktop layout starting in about 1986, got really good. Page Maker, Frame Maker, Quark Express... and the introduction of the PDF format, replacing the machine inspecific PS and LaTex formats. (Well, PDF is technically a zipfile including PostScript, so, PS is still with us, hidden inside.) Computerization of the flow made design so much easier. (There were some not so good layout programs, too... Print Shop. It was fun, not good.)
There also was the induction of metacurrencies past HP and XP, also in the 80s. MSH had Karma as a metacurrency in 1983. WFRP 1e (1986) added Fate Points. 1991 gave us Willpower in VTM. While their being useful is argued, they brought in new approaches to play, and the VTM Boom was a lot of people brought into RPGing who were not interested in the big 3 visibles: D&D/AD&D, Star Wars (1987 WEG), and Star Trek (1983, FASA).
If nothing else, newer designs have better tools to convey information to users. The many clones of OE D&D are almost all more pleasant to work with... even if the mechanics are the same as OE D&D.