The answer to this question can be found from the businesses that choose such systems. First, most of the businesses that end up getting a new digital phone system for their office start by exploring the latest technology opportunities that are available, VoIP or an IP-PBX in-house system that can connect through any means (VoIP SIP trunks, CO Lines, or PRI). These businesses are small offices or retail establishments that may only need 3, 10 or 15 phones and are operating with traditional copper phone lines, probably not a PRI circuit. They quickly limit their new choice to either an over the open Internet VoIP service (hosted business VoIP) or a small new digital system, due to the fact that the initial up front cost per phone can be significant when getting an in-house IP-PBX. Unlike a larger company that wants later technology, special features and makes budgeted purchases based on their future needs, which is often spread out among 40 or 80 users, a very small company is looking primarily at price and that price is going to be substantially lower for digital phones connected to their already in place in-premise telephone lines. If cables need to be run and the LAN is not VoIP ready, costs increase.
In the last two or so years hardware costs of a VoIP ready Asterisk based IP-PBX and SIP phones are half of what they were just a few years before. Now a later technology IP-PBX appliance with SIP phones can almost be in the same hardware price range (installation costs vary). The buying decision often comes down to how comfortable a small business owner feels in managing the system. And how good is the cabling on premise and LAN infrastructure (routers, switches, etc.). Companies with digital phone systems in place that need to be replaced typically have infrastructure that is the same as when the system was originally installed. Unless a new phone or additional line was needed, it remained mostly untouched. Cabling is might be phone wiring using RJ11 outlets with some phones in places where there is no computer.
The caveat may be very small companies where advanced features would drive better revenue, like a real estate office, attorney's office, etc., or companies with significant call volume.
Simplicity and familiarity also play a role with some companies that get a digital phone system. They look at the cost, ask a few feature questions and quickly find out that it will do more than their old phone and can connect to their existing phone lines, with no need for porting, cancelling contracts, or moving carriers. If an alternative Asterisk based system is offered at prices that compete with any other system, and can be connected right to their existing lines, the decision may often rest with the purchaser believing simple is better; i.e. fear of too much advanced technology.
Many small companies have traditional phone cabling and limited older network hardware. When confronted with needed upgrades for an IP phone system they start to rethink the purchase.
Choices for a new digital phone system for a very small company that include voicemail and an attendant can go as low as $1300.00 to $2,500.00, plus installation. This low price, unheard of five years ago is grabbing the attention of both business owners that need to replace a failing system. Older systems that are replaced due to a part going bad or possible taking a voltage surge during a lightning storm are rarely budgeted for and if the company can place the loss on their insurance, they typically need to make the case that the older system is too antiquated to be fixed and the new system is similar.
They are the small businesses that everyone knows, pizza franchises, hardware stores, lawn service companies, insurance offices, parts stores, and all the other traditional small existing businesses that have two, three or even four phone lines from a traditional carrier like at&t, and a fax line and have around four or five or six employees. Phone lines are in place, already connected to a block in the phone closet. Some phones may be in places where no data connection (computer) is present and the cost to run one, two or more cables would add up.