The Parker Solar Probe will make its closest approach yet to the Sun

Look at this NASA animation of two solar probes orbiting the Sun (thanks ostacke and DiggyJohnson):

https://svs.gsfc.nasa.gov/3966/

One probe, Parker I assume, goes through all the planetary flybys to achieve its solar orbit. The other just drops into an even closer solar orbit. Why not do that for both probes?

The Parker solar probe gets much closer than the solar orbiter, 0.046 AU vs 0.28 AU respectively. The successive Venus flybys are to drop it increasingly further into the sun's orbit to take solar atmospheric data on quick flybys while the Solar orbiter is more for spectrograph measurements of the sun's corona, just different mission sets.

You can't "just drop", it requires energy (fuel) and that's probably the answer why.