There are a few reasons that might explain why.
Power discrepancy is probably the most important reason.
And with that, companies have to decide if there's enough reasons to try and port/bring a multiplat. The userbase (demographics, mainly) is there but, if we look at the data we have for the US, only 30% of Switch owners don't own a PS4/XB1. That alone makes porting for Switch less viable (those same 70% users already have the option to buy the better version and probably at discount price).
When we look at the games we have from 3rd parties, things are not that encouraging either.
If we exclude M+R (because of the Nintendo characters), Fifa 18 is the (second) best selling 3rd party game with 1.03 million, followed by Skyrim with 1.01 million (a game that everyone has played a gazillion times) sold.
In third, we have Project Octopath Traveler in with sales of 0.66 million.
And looking down things just get worse.
Now, if the userbase wasn't there, we could say that lower sales happened because of different demographics, etc.
And, we can't exactly say because it's ports or remasters, when Fifa 18 sells a little above a million to 20 million Switch owners.
Even Sonic Forces is above Doom, and not just by a little.
Honestly, the reason might just be that there's no real BIG incentive to buy ports or multiplats on Switch, at least, those that are not Nintendo-like titles.
Though, all of this can change over time.
As some have said, games take time to make, but the annoucements, don't.
If the games were coming we would have known most of them by now.
