Two reasons.It's puzzling that the Swiss watch industry seems nowadays to have lost interest in advanced technology.
Either they can't compete with the Japanese. Not much fun having a watch performiong at +/- 25 sec/year watch when the Japanese has +/- 5. Then Swiss watches will appear inferior and the "Swiss watch" brand will loose it's branding value.
Or they have much less control over the components going into the watch. Making a mechanical watch in-house does not require atomic age knowledge and technology. However making a quartz watch 100% in-house is very demanding. You have to grow your crystals (high pressure and temperature) and make your own chips. While the chips are not 20 nm or what ever Intels latest is but even making chips with the "large" mask size that was used in the 80s (and which is quite sufficient for even complicated digital watches, temperature compensation and so on) is very demanding in both equipment and knowledge. So if you're not satisfied with buying crystals and chips from the large components manufactures manufactures to your specifications - well then it's either extremely expensive to make a quartz watch or impossible.
So while still making ordinary quartz watches to full fill the demand for cheap watches they've mostly concentrated on the mechanical niche marked which has seen a great revival in the recent years. If not the marked has grown from niche to relatively common.
But my guess is it may only be a question of time - at least if the trend is people want a watch and not just look at their phone. I myself prefer a watch. Much quicker to look at than the phone let alone all the functions a modern multi function watch offer which are deeply buried in menus on the phone. But with that i mind I think the trend may turn at some time. People go back to quartz (or perhaps soon MEMS) when they're tired of their mechanics precision. While I do have mechanical watches and I love them I don't use them for daily use - too much hassle of setting them frequently, winding etc. so I only use them on special occasions. That's also a thing i miss on the mechanical ones. While a stopwatch is obtainable at reasonable price - a timer or alarm is not and while as said functions are on the phone they're often a hassle to use while on the watch the functions are at your wrist.