Difficult to understand why varroa kill hives. There are so much sure advices how to handle varroa.
I believe there are many reasons.
1) A lot of new beekeepers don't understand the cycles of the varroa and wait until it is too late in the season and find themselves vastly infected when it it too late to overcome.
2) A lot of "wishful hoping" on ineffective treatments. There yearns that desire to find the ultimate "so-called natural" treatment. That sometime appears to be working because the user so has there heart set on it. When you don't have varroa epidemic, any treatment will appear to work.
3) Seems like a lot of folks need to learn for themselves the hard way.
4) I also believe a lot of varroa killed hives are chalked up to "bad weather". WHich is unfortunate since they don't change their ways in the future. I cn remember growing up (pre0-varroa and pre-global warming
)and my Dad had 100 or so hives, never did any special winter treatment (wrapping, entrance reducing, etc) and on a bad year he would loose less than 10%. Now a days you hear of 40,50,60% losses.
Since switching to oxalic acid 3 years ago, my winter losses have been less than 5%. I also have 2 TBHs that I did not treat at all last year and they are blistering this Spring. I know you are not a believer in natural cell size, but I will continue to try it on a few hives as Michael has described.