Just kinda thought about this today... (Although for the most part, I do like the Customer Portal)
They had a system in place prior to the acquisition. It would have been difficult I'd imagine to implement an entirely new system after the one they had was already set up to their needs.
So you're saying that it's difficult to migrate to WHD?
No, but I know what system they had (having previously worked there) and with how in-grained that system is company wide (I won't get into specifics obviously) it's not as simple as just migrating to a new support ticketing system. There would be much more involved for them to do it. Migrating to WHD isn't actually that difficult having assisted clients with it when working for a SolarWinds services partner previously. It's time consuming, but not difficult.
Not a silly question at all and you are not the first to ask either . mharvey is correct in many of his comments, but let me expand upon them a bit further. First, let me start off by saying we do use Web Help Desk or WHD for our internal help desk to handle all of our IT, Facilities, HR, Internal Apps/Systems, etc. needs, so as they say "we are eating our own dog food". Now back to the original question of why do we not use it for external support use and Matt hit the nail on the head. We have spent considerable time and resources both customizing and integrating many of our systems. A basic and small example of this is the integration of our support system with our internal bug and enhancement tracking systems. It is also tied into other aspects like sales, order entry, licensing and other areas I am sure I am missing. We also have well defined processes and procedures that have been built over the years as well that would have to be tweaked with the introduction of any new system. I hope this sheds some better light into this and if not, please ask and I will answer the best I can.