Panzer is correct; most Trail maps are double-sided, which would leave little or no spare space for town information.
Also, the maps are designed and published by local Trail Clubs, and most of these folks probably don't want to get into providing town info on the maps as there would be inevitable arguments and disagreements over which places to include and which to omit. In some cases, trail maintainers and local club members actually own some of these hiker facilities, and in some circles it might not be seen as proper for the maps put out by these clubs to be seen listing (i.e. "endorsing") certain facilities, or omitting others. Lastly, the map sets for each particular section are re-done and revised every four or five years, so maps that had Town facilities and businesses printed on them would inevitably contain false or out-of-date information (like places that were no longer in business) and would also be incomplete as they wouldn't reflect new businesses either.
The great advantage to the standard Trail Guides is that they're constantly being up-dated and revised, with obsolete information hopefully removed, and new information added. This is especially true with the Thru-Hikers Companion, where each section of the Trail is re-vised every year by someone who either lives in that particular neck of the woods; is intimately familiar with the section, or has just finished hiking it.
In short, the standard guides contain an extraordinary ammount of useful information, which is why most thru-hikers elect to carry and use one or the other.