It's probably not one single reason. Tesla has had a lot of good news lately, from reports about Elon behaving a little better (and winning that defamation lawsuit), to competing EV from companies like Mercedes and Audi not really selling well and securing a large area for GF4 near Berlin. The most impressive feat for me was how quickly they built GF3 and started production in China, but my tiny contribution to the stock price was earlier this year (late May), the return to profitability in Q3 probably impressed many investors as well.

Other smaller stuff too:

  - Porche's Taycan EV is supposed to be the closest direct competition for Tesla cars, but its having production issues.
  - Tesla just pushed a new software update that includes a lot of useful features (voice keyboard/commands, an official camping mode, saving camera clips on honk, etc)
  - A lot of miscellaneous positive press is cropping up from the end of the year and end of the decade type reporting - Time for example put it as one of the "best gadgets of the decade".
  - They recently released and presumably are testing a paid software update to boost M3LR off the line acceleration, which is an interesting situation to obtain revenue from already sold cars.
  - Internally they're pushing to sell a lot to break sales numbers before EOY.  Some places are reporting that they're even selling showroom floor models.
  - They secured a US$1.4B loan from Chinese banks for GF3 acceleration, which was already ahead of schedule.
  - They closed on the land for GF4 and are reportedly looking to start building Jan 2020.
  - The recent news of possibly reinvigorating the EV tax credits in some way probably also helped.
  - Solar is ramping up as well.

Stock prices are based on perception of where it's going. The surprise profit from the last earnings, trade deal leading to lower tariffs, new China factory opening up that market, and the general quality of electric car competition released this year have all shown that Tesla is still far ahead of the game and close to running a sustainable operation.

There have been 5 or 6 different pieces of good news in the past two weeks which have led into what I believe is an insane enthusiasm for purchasing this stock. Go on investing forums and you will see people casually throwing out numbers like TSLA to $1000. It's like bitcoin 2.0. I would not short it.

There has been lots of positive news in the past couple of months, and you'll see some listed in other replies, but IMO it's more due to the fact that the bear story has disintegrated.

Prognostications of Tesla’s doom have gone from concerning, to annoying, to boring, to pathetic[1]

Tesla has a successful ongoing business just selling Model 3's and Y's. The S could be destroyed by the Taycan, the Cybertruck could flop, the Semi could fail to take off, Autonomous driving could remain a slightly more intelligent cruise control, their solar tile business could fail and Tesla would still be OK. It wouldn't be worth $420 a share, but they wouldn't be bankrupt either.

1: https://cleantechnica.com/2019/12/21/prognostications-of-tes...

Musk was able to get funding in China for the Gigafactory there. That's my best guess

https://techcrunch.com/2019/12/23/tesla-lands-1-4-billion-fr...

Its probably because of the pickup truck that is priced very nicely (but don't rely on my personal opinion)