I would say the smartphone market would be more stable in terms of market share by its nature. The only way that it would shift significantly is if there were a mass exodus of users from one platform to the other. But the vast majority of people are perfectly happy staying with whatever they have. Likewise, many new smartphone users who aren't locked into a platform yet get locked into whatever platform people around them are using.
In the vehicle market, there's a lot more competition space than just two or three brands. And just because someone is around, say, Ford drivers. It doesn't necessarily mean that they're going to go out and buy a Ford for themselves. Rather, they're going to buy whatever they find appealing.
Taken to the extreme for example, say Apple had 100% of the market. If a competitor entered the market and gained 1% share,then Apple's market share will have decreased.
In Tesla's case, a number of competitors have entered the market. It would be impossible to maintain whatever market share they had no matter how good the product is or how well their stock price is doing.
In the vehicle market, there's a lot more competition space than just two or three brands. And just because someone is around, say, Ford drivers. It doesn't necessarily mean that they're going to go out and buy a Ford for themselves. Rather, they're going to buy whatever they find appealing.