Not surprising. Thinness is not as big of a priority for most consumers when compared to battery life or cameras. Most people want either 1) the basic phone with most of the features, 2) the flagship with all of the features. It's hard to justify a phone that is more expensive than the base model but is missing some of what the base phone offers (more cameras, better battery life). Its thinner, appealing design was supposed to compensate for the missing features and the higher price, but it doesn't appear to be doing so.
Apple hasn't been able to come up with a viable "fourth option". The Mini and the Plus were both the least popular of the models when they were available. They tried smaller, they tried bigger, now they've tried thinner. Not sure what else they can do to make something as successful as the base model and the Pros.