I dont imagine there is a big enough market to justify it. You can thank greedy network providers for that. The vast majority just tether instead of paying for yet another contract.
I honestly don't understand how LTE capability has any bearing on something being considered portable or not.
I find it hard to believe that there isn't enough market to justify making an LTE surface. Especially if there is enough market demand to justify the Surface 3 LTE. I would expect that enterprise users would easily eclipse the consumer demand for the Surface 3. My company as an example would purchase the LTE enabled model just in case it would be a field requirement for our staff. I would expect this would result in around 75-100 surface pro 4's being purchased with the LTE option and an additional 50-75 surface pro 4's being added to our stock replacing iPad tablets currently in use by those requiring a field device with data connections.
Microsoft is missing the mark here if they are holding back an LTE model due to concerns about demand. We would happily purchase SP4's as they would converge a PC and an iPad for our staff. The added cost of LTE is offset by utilization of one device and would result in an overall savings to our company.
I also understand the comment from SetTrend regarding portability. For those of us who demand a full time data connection, adding a USB dongle or hotspot is inconvenient, reduces the full time connectivity of the device and is one more device to lose break or charge. Alternatively, using your phone as a hotspot presents the same problem regarding constant connectivity, diminishes your battery quickly, and in certain cases prevents you from using your phone simultaneously.