They are not that good now but they were excellent back then. Modern TFs benefit from all the technological development that has gradually occurred since G1/Diaclone/Microman (well at least most of them)
They were excellent toys when they were released and therefore continue to be excellent because in that context, they were far ahead of their time.
Its not much to do with nostalgia but rather when they were made. The Roman Colosseum doesn't look that great when compared to modern stadiums but it was a an awesome architectural achievement with the technology they had 2088 years ago. Just because in the 20/21st century we can make bigger and better stadiums, it doesn't make the Colosseum anything less than a gargantuan achievement.