Because some leave either elements or deep registry entrys that aren't removed on the automatic uninstall. When avast installs it checks for other resident AVs, if found avast may give the warning/error message and refuse to fully install all the avast elements to avoid a possible conflict, which could cause your system to freeze.
Two resident scanners are likely to be in conflict when a virus is detected which AV deals with it? They are both likely to be fighting over it.
You can either us a non resident scanner as a backup and run periodic on-demand scan (with the resident scanners disabled) for a second opinion or use one of the many on-line scanners (not Panda as this is likely to have avast throw a fit on your next avast scan because it doesn't encrypt its virus signature file).
RejZoR's Website - Security Ops
On-line Virus Scanners and other useful Links
Security-Ops.eu.tt
