Because Santa Barbara, SBA, the airport local to Montecito, doesn’t have customs facilities, it’s a domestic airport only.
Aircraft inbound from the likes of Canada have to first make a stop at a customs equipped airport (e.g. Seattle) for clearance before continuing on to land at SBA.
C&P (hence the capitals, sorry) from notes on SBA facilities, effective 23rd Jan:
“NO CUSTOMS PSNL OR FACS ARE AVBL AND INTL ACFT WILL NOT BE ALLOWED TO LAND UNLESS AN EMERG EXISTS.
https://www.airnav.com/airport/KSBA