Because Token Ring frames don’t allow for more than three multicast MAC addresses.
From the Cisco documentation:
HSRP Standby IP Address Communication on All Media Except Token Ring
Because host workstations are configured with their default gateway as the HSRP standby IP address, hosts must communicate with the MAC address that is associated with the HSRP standby IP address. This MAC address is a virtual MAC address that is composed of 0000.0c07.acXX where XX is the HSRP group number in hexadecimal, based on the respective interface. For example, HSRP group 1 uses the HSRP virtual MAC address of 0000.0c07.ac01. Hosts on the adjoining LAN segment use the normal Address Resolution Protocol (ARP) process in order to resolve the associated MAC addresses.
HSRP Standby IP Address Communication on Token Ring Media
Token Ring interfaces use functional addresses for the HSRP MAC address. Functional addresses are the only general multicast mechanism available. There is a limited number of Token Ring functional addresses available, and many of these addresses are reserved for other functions. These three addresses are the only addresses available for use with HSRP:
c000.0001.0000 (group 0)
c000.0002.0000 (group 1)
c000.0004.0000 (group 2)
Therefore, you can configure only three HSRP groups on Token Ring interfaces, unless you configure the standby use-bia parameter.
You may wish to refer to RFC1469 – IP Multicast over Token-Ring Local Area Networks to fully understand Functional Addressing.
FCoTR lovers need not be concerned at this limitation on the use of multicast is not generally needed except for very simple end point discovery mechanisms. Since FCoTR networks are commonly dedicated storage networks, three multicast addresses are more than adequate to perform auto-configuration discovery requests on a FCoTR network.