Because, volume.
When Tesla started Model 3 production, their plan required them to build a battery factory which had the same capacity as all other lithium ion cell factories produced globally a year before.
Tesla has a somewhat genius business model from that perspective.
Make a great car and sell
loads of it.
Somewhat like what Apple did in the early 2000s when Jobs scrapped most of the legacy products and set a simplified product line policy of having one consumer laptop, one pro laptop, one consumer desktop, and one pro desktop, each with a couple of variants.
Build them and iterate on them to make them insanely great and then over time as they figured out what works they expanded in to other products and a much more rich set of variants.
If you look at competitors who do have loads of models they’re not selling anywhere near as many of them, nor are most actually making money from them. Ford have admitted they’ll be losing money on their EVs for years which they can fund from profits elsewhere in the business. VW stated they’re suffering poor EV demand despite the complex lineup so increased prices on petrol cars to prop that side up. GM is throwing models at the wall with ridiculous features and equally
pricing to match, yet are still struggling to build more than a few hundred of each month.
Buttering yourself too thinly leads to suboptimal user experiences, lower sales, lower profit, and eventual bankruptcy. Especially when you’re still somewhat new to making cars and are figuring out how to do that cheaply, sustainably and quickly, and ramp up the supply chain to support it.
Right now battery tech available and the energy density of cells means a compact car from the competition is getting about 250 miles of range, which for many consumers is perceived as a downgrade from an dino-juice car.
So they’re iterating on what they have, focussing on getting the most out of what supply they have by for example increasing the range of the Model 3 by over 100 miles since it launched 6 years ago and they’ll soon package those learnings in efficiency and aerodynamics in to a new smaller car that handily beats anything else.
No point in releasing a suboptimal product too early when your sales are sustaining the growth you can support in terms of the battery supply you have available with just a few models, complexity is overrated.
Oh and don’t forget the issue of having to build factories from scratch. Any new variant takes away capacity for what there is today, is it worth selling fewer Model 3 by converting a line in Fremont to something else? Not really. Hence the newly announced factories in Mexico and planned expansion of Berlin and Nevada to finally start making the smaller cheaper car with all the economies of scale to make it competitive.
Elon quite helpfully laid this plan out way back in 2006
