Because it's completely unexpected and has a long tradition of being a PS franchise. It's just one example of things that'd get gamers knickers twisted and them getting excited (positive and negative). MS buying the rights to Uncharted would be a similar megaton event that'd make a presentation worth watching.
Or MS announcing a new handheld completely out of the blue that doubles as a Windows phone.
Or MS announcing their own VR headset.
Lots of big ticket, improbable events could happen at any of these presentations and they're the ones that get forum posters writing excitedly. Some people are going to be enthusiastic with some upcoming game announcements, but the majority of us just aren't that into it. Doesn't matter who's showing.
that's disingenuous. The posters who ramr was responding to were saying they personally weren't enthused by the games MS showed. The same games on PS4 would have the same response from the same posters (save the few fanboys). If Sony doesn't show the games that'll interest people, the response will be suitably muted. A VR showcase could prove very dull as it'll be video of someone else experiencing the joys of VR. Only if there are some exciting VR games announced, or a shockingly low headset price, will gamers respond excitedly to a VR show.
Remember that Nintendo 'won' E3 last year because their show was the most entertaining, and had unexpected newness, like Amiibos.