in my opinion, the main reason why I keep using same exchange just because the best exposure to liquidity and good services it offered aside from the fact that its easier to do accounting on our end for tax purposes as well.
i'm pretty sure people who just stick with one exchange is because out of convenience, they only go out of their way using other exchange possibly lesser known ones when they trying to trade ticker that aren't available in their main exchange.
but once that ticker available and listed in their mainly used exchange they will immediately come back.
So, why do prefer to consistently use the same exchange?
If an exchange has not given you any reason to doubt their services, there is no need to switch to another exchange. Being a long-time customer of an exchange can increase your customer status with the exchange because they know that you are a long-time customer, so you may have some kinds of benefits.
Using many exchanges at once is a security risk to your details and to whatever amount you have kept back on that exchange.
funnily enough most of the exchange i know that are top ranked never truly appreciate long time customer, they always give promos to the new customers, the long time customer can still join certain event but most of the event still focused on the new users for the purpose of increasing their users.
it has always been like this new user get incentivized more.
so I can see the reason why some people decide to just use many exchange to pursue the discounts and promotional offers, but it is one time event anyway, since afterward they're not eligible for all the promotional offer anymore.