for myself personally it's because I love gaming on a big screen (use a epson full HD projector @ 160") and I love portable gaming, and until the Switch appeared the best portable device going was the Vita which had the same amount of support as a castle built in a swamp. Don't get me wrong the Vita is an amazing piece of hardware and I feel it does have tons of great games, but they're all just wannabe ports of their big console brother counterparts, there are very few direct ports of titles from the other consoles, when it comes to buying software on the Switch in my head I almost feel like it's all half price because before I would have been buying the ps4 and vita versions of a game (if crossbuy wasn't on the title) and at the same time I would be getting a far worse version of the game for the handheld. On Switch a game is 60e but for me it's a 2 for 1 deal compared to last gen, it really takes the sting off getting a new game when you see it like that.
Also the reason that Switch can easily sell more with slightly weaker or what would be niche titles compared to those same games on the WiiU is easy, the system is selling like hot cakes and already has an install base of more than double what the WiiU sold over its whole lifetime, a game can perform half as well on Switch and it will still outsell its WiiU counterpart. The success of the machine is why you're seeing more and more third party support and now there is less need of Switch port begging and we even see companies who had said "fu" to a possible switch port like Hat in Time are now eating some humble pie and putting their game where they know there is money to be made.
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