Another reason is that the alternative of syncing your device with a computer doesn't work with most of the population. They either aren't familiar with tech enough or they don't own a computer. The rest get annoyed with the lack of convenience vs automatically uploading everything via your phone's bluetooth
Edit, may not be entirely laziness: see this response to my question - https://news.ycombinator.com/item?id=25787066
Also, I seem to remember long syncs timing out so many times in a row that I just had to factor-reset my FitBit HR to clear its data buffer. This happened more than once. It really needs an incremental sync.
When I got my FitBit HR, my wife was interested in using my original FitBit. I proceeded to update its firmware right before I gave it to her, but the firmware update seems to have timed out and broken the FitBit. I recovered once with a factory reset, but a second firmware update attempt bricked it.
Honestly, IoT devices need minimal firmware in non-brickable ROM that checksum the firmware in flash before jumping to the firmware in flash. If the checksum fails, then make a light blink and go into a recovery mode that implements a minimal Bluetooth or HTTP firmware update, preferably incremental, requiring minimal state, and recoverable/restartable if that minimal state gets corrupted. Back in the day, I bricked a WRT54G router by using Firefox to upload the firmware instead of Safari or I.E. (It was a known non-deterministic issue, apparently. I'm guessing a corner-case in the router's handling of chunked HTTP encoding.) Not even the trick of shorting two adjacent pins using an x-acto knife could get the recovery TFTP server to come up.
Also, during the first year warranty period, my FitBit HR died twice, presumably due to being splashed so much with salt water during dragon boat race practice. I know it says not to go swimming with it, but you should really avoid excessively splashing it with seawater. When my FitBit HR died a third time shortly after the warranty period, I gave up on FitBit for a while.