Upvote:2
The electors were likely looking toward the future, anointing Hendricks as future (Vice) Presidential material. Hendricks was a Hoosier of national prominence who could help the Democratic Party make inroads into the North in 1876. Benjamin Brown retired from politics after the election, so there was no need for the electors to defer to him. And anyway, Brown was from Missouri which was so solidly Democratic that it went for Greeley by 16 points even in the Republican blowout of 1872.
By contrast, Indiana was the swing state of the era, giving it the potential to decide a close election. Look at the list of close states in 1876, 1880, and 1884. All three were tight elections in terms of the popular vote. Democrats narrowly won Indiana in 1876 and 1884 (with Hendricks on the ticket) and narrowly lost it in 1880 (without Hendricks on the ticket). Indiana was the closest state Democrats won in 1876 and the closest state they lost in 1880. Hendricks was quite literally the difference maker.
Ticket balance explains a lot, but factionalism also would have been a factor. Greeley/Brown were originally nominated by the Liberal Republicans. The Democrats only reluctantly supported Greeley in order to avoid splitting the anti-Grant vote and going down to an even more certain defeat. The uneasy Liberal Republican-Democratic fusion dissolved upon Greeley's death, freeing the un-pledged electors to shift their support wherever they wanted. Likely, it was the Democratic electors who voted for Hendricks over Brown. Hendricks had been a Democrat throughout his political career (never joining the Liberal Republicans), whereas Brown had helped found the Republican Party in Missouri. Solid Democrats in the electoral college would have preferred supporting the solid-D Hendricks over the uncertain Brown as a simple matter of party loyalty.