Here's probably why.
The Tesla Model S dates back to 2012 and received a new body-color front fascia for 2016 to match the design of the then-new Model X. Since then, neither vehicle's exterior or interior designs have changed, though both have received numerous over-the-air software upgrades. Given the rapid pace automakers are refreshing their vehicles lately, it'd make sense for Tesla to do the same, especially with new EV rivals from Audi, BMW, and others already on sale or are coming soon.
According to Electrek, it appears Tesla is quietly up to something in this regard. The California-based automaker has confirmed it plans to shut down both Model S and Model X production lines during this holiday season for an extended period of time.
An email sent to employees last week stated the factory shutdown will last from December 24 to January 11. On the one hand, a holiday closure is not unusual, but an 18-day shutdown is for any automaker. Employees are also receiving an extra week of paid vacation during the shutdown, but the email hinted at the possibility there will be some overtime possibilities to assist other teams and deliveries. The email made no mention of vehicle refreshes.
Not long ago we reported major Model S and Model X updates could be coming soon following the revelation of some significant price increases of around $6,000. To justify that, Tesla will need to offer customers more than just software advancements.
Based on the information available, Tesla has some major interior design changes coming for both vehicles. These are were initially planned for summer 2019 but Model 3 production took priority. Now that it's been sorted out, attention is now back on the aging Model S whose sales have dropped over the past few years. The sedan and mechanically similar Model X will reportedly receive exterior design changes as well and the timing works well because the new Model S Plaid also requires numerous design changes.
If Tesla is indeed planning a significant Model S and Model X redesign, official debuts could happen very soon. Shutting down the factory over the holidays for 18 days is the perfect time to do some major re-tooling.