agree with aim. rpm's.
oil injection, cdi ignition (mid '70's if i remember right), better machining tolerances, lined cylinders etc. could be others.
EPA..... indirectly maybe. there was leaded gas available up here near me to about 1987. once the lead was out, (that was when gas was 99 cents or so), think most gas had MBT (or something letters starting with a M like that), that ingredient was supposed to lubricate and protect like the lead they took out did. the ingredient caused red fouled plugs in the long run and was eventually banned (thinks me) supposedly for killing fish and everything else for that matter. hopefully everyone knows about the danger of long term lead exposure these days. both of which are bad things in my book.
most folks, including me running 2S oils like castrol, valvoline or belray. most all 2S oils of 20 years old were composed of brightstock and napthnenic oils + sulfonates of sodium and calcium. WTH?
(sulphur BTW does not particularly like nikasil).
newer 2S oils also contain some zddp. (zinc/phosphorous). which are bad for cat converters, but good for anti-wear. most everything smoked 20 years ago. if it didn't, it got you worried.
these days with the carbs smaller stock jet openings, cats in newer saws, unleaded fuel, smaller stock jet openings, and the like, could likely are the reasons.
most synth 2S oils are a waste of money for anyone who burns less than 5 gallons through one saw in a year anyway IMO, (unless they run higher rpms, or continually cut with dull chains, or run ludicrously light oil ratios). the average folks need mineral oil which fights corrosion better, which is more important when the saw is pulled out only a seasonal basis. (to me anyway).
today's carbs seem to be jetted leaner (as folks say on here anyway), if you want to run the fatter oil ratios, hope folks adjust accordingly, because they are not set up that way stock. much easier to run whatever ratio manufacturer recommends.
-OMB