The Hostcount collects its data via zone transfers. Many organisations are increasingly wary of allowing zone transfers of their domain for security reasons. It's also possible that some organsations wish to restrict zone transfers due to the increased load on their nameserver.
The impact of restricting zone transfers for a zone is that all the data about the zone and its child zones is lost.
The two primary reasons for major decreases in the number of hosts counted for a TLD can be identified as:
Major increases in the number of hosts occur when there is no restriction.