Probably because some over-paid advertising executive was desperate to find a way of convincing the manufacturer that his company could successfully relaunch the brand.
To be slightly less cynical, there might actually be some valid market research behind the name change. If you were to go out into the street and ask people what the word 'bounty' means to them, most people would probably mention chocolate bars (with a few possibly also mentioning mutiny). Very few people would define the word as 'a munificent gift'. On the other hand, if you were to ask people what the word 'plenty' means to them, they'd say things like 'a lot', 'loads' or 'more than enough'. If you want to suggest that a kitchen roll is very long, and can mop up loads of liquid, 'Plenty' would seem to be more appropriate than 'Bounty'.
Chris