As to why this change occurred, it is likely the result of the lighting industry slowly, over time, merging photo bulbs into a smaller number of replacements. Since photo bulbs are far less common today than they were 30 years ago (exhibit A = price), and they sell far fewer of them, it makes sense (to them) to squish 1000 different bulbs into 200 or so "compatible" varieties.
It goes back to the fact that it's cheaper to offer 200 varieties than 1000 of them. Now, in it's heyday, the sales of these bulbs justified the cost of producing wider varieties, that's just not the case anymore.
Brian Dancer
Service Lighting
