I think 110's were quicker to be scraped and / or put out of use because newer models were available that took up less space.
81's hung around a bit longer, and because they are a bit smaller (not too much) they were tucked away in a barn / garage / back room more so than the 110's were.
And because the 81's were used longer (in my opinion) when it did come time to replace them, Coke, 7up etc... never took them back when they gave the gas station a new one as it was forgotten about or just not worth the expense to take them back (assuming it was given to them by the bottling company). If they were outright owned by the sellers.. then it all holds true the same.... 81's were used longer and easier to tuck away.
If anyone else has an idea as to why there are not more 110s around, would like to hear your reasons