From a collector that appreciates the history of Pokemon and its journey through the years, vintage cards certainly deserve higher prices in today’s market as there is a unique eye appeal to them that invokes memories and nostalgia to the beginnings of the hobby; this by itself brings a form of value that modern is unable to provide. From an investing perspective, vintage cards mostly grow at a steady pace aside from stuff like your gold stars etc, because they get scarcer by the year and availability slowly trickles down; modern on the other end experience very volatile movements and experience different waves of hype due to changes in collector preferences and heavy market manipulation.
With that said, if it is part of anyone’s plan to grow their collection at a possibly quicker pace, and they have the capacity to handle the volatility of the modern market; to completely give up the modern market and going all-in for just stability in vintage might be giving up too much of an opportunity cost. Having stake in the modern game allows one to understand what the masses in the market today are thinking, as people tend to put in efforts to understand and study prices based on what they own. Over time I guess this can help us adapt a more neutral mindset when viewing the market through a different lense.
As for why vintage overall is still progressively slower in terms of growth at least for the past couple years, I personally feel that lesser and lesser people (especially newer collectors) are making that distinction to collect either purely vintage or purely modern. As such, without any specific preference they would usually end up with the more easier to access modern cards; not to mention vintage wotc PSA 10/old back PSA 10 of popular pokemon usually have a higher barrier to entry; and a wider percentage of people in this hobby still prefer to see a 10 on their graded card. When people have a stake in more modern stuff, they subconsciously gravitate towards it over time. Nowadays I am heavily invested into both vintage and modern; as I dont see why I couldnt just own both a 1st ed zard and some copies of the 151 SIR zard (saw rusty purchasing some 151 SIRs from his video not too long ago as well), as well as some corocoro mews for vintage and bubble mews for modern. Best of both worlds 
v.