Probably because they turn off transponders while crossing that area.
"Traffic is generally picking up in the strait, with several laden tankers seen exiting into the Gulf of Oman over the weekend, though some of them turned off their transponders. The latest was a Greek-flagged tanker carrying Iraqi crude to Singapore."
"Traffic is generally picking up in the strait, with several laden tankers seen exiting into the Gulf of Oman over the weekend, though some of them turned off their transponders. The latest was a Greek-flagged tanker carrying Iraqi crude to Singapore."
https://financialpost.com/pmn/business-pmn/iranian-crude-oil...