There's no technical reason why an LTE network can't be engineered to provide truly unlimited data with acceptable speeds in most instances. There is, however, a financial reason, plus the usual regulatory/political concerns that get in the way of new cell sites. It's worth noting that T-Mobile manages to offer unlimited with an asterisk (video throttled to 1.5Mbps) and in many cases delivers superior speed than Verizon, so it's clearly POSSIBLE and PROFITABLE to use as a business model.
In rural/fixed-wireless settings LTE is actually cheaper than DSL/cable and the favorable contention ratios (i.e., low population density) make unlimited possible with today's network. It's a mystery to me why they won't offer an unlimited product for this market segment at least; it would be the death blow for satellite internet.