Why are so few heliports depicted on sectionals?
I would disagree with the statement that pretty much every grass strip is charted.  This is common in CONUS, Alaska and Canada. There are at least 30 within a 10nm radius of mine which are not. The reasons for charting grass strips are usually: 
1. You are inviting people. 
2. You want to notice people that the strip is restricted. 
3. You are in an area, where charting your strip for emergency use is a public service you choose to offer. 
4. You have tenants on your strip, and you want to ease their navigation and those who may visit.

As for heliports being charted: If they are intended for public use, they frequently are.  They are also charted if they have allot of traffic (eg Hospital / EMS / police barracks). Many private helipads do not wish to invite the public, and there is no purpose in charting them.