This was likely done to keep Musk from being stock on the market at a reduced price prior to purchasing the remaining needed shares at the prior price, thus lowering his overall costs. It's a tragedy of the commons situation where any individual shareholder would be incentivized to sell early. It's understandable why they did it, but really insider trading (like most other rules) is over-enforced on the masses and selectively not-applicable to those who wield power.
GM Finance used to have a deal where they'd virtually sell your locked-out shares in anticipation of a market bump after the lockout. Is that still a thing?