Poplar is somewhat harder and more dent-resistant than pine. As a species, poplar is not inherently more stable than many sorts of pine, but the quality of poplar boards available at your big-box store may very well be higher than that of the pine they carry. A lot also depends on construction methods, but your pics aren't good enough to make out the details of what you did.

The main problem with any wood is the moisture content and the structure of the grain. Pine is a soft wood that warps easily, is difficult to dry evenly, and can have a lot of stress in the structure---meaning it can warp even as it is being cut more than most other woods. Working with pine is more difficult in many ways than working with most other woods---pine is softer, has a LOT of pitch/sap(which coats saw blades/etc.) and is difficult to stain evenly.

If you glued individual boards together to make the top that warped, you should have alternated grain patterns---that reduces some of the warping tendency. Also, each board has to have the same moisture content and be dried the same way---air dry or kiln dry. With newer pine lumber, even the grain spacing need to be matched to a degree---pine is fast growing and is harvested sooner that it used to be---meaning the growth rings are wider, which increases warping probability.

Table tops should be attached to the skirt/base in a manner that allows the top to move. Fastening the top securely to the skirt/base will not allow the expansion/contraction to occur---and the wood warps.

Poplar is called 'Painters Wood' due to the smooth texture of the sanded wood. Poplar also has two very different colors---white and green----often in the same board----which makes the wood not worth staining. It takes paint very well.

Maple is a popular light colored wood for furniture, walnut is the common dark choice. Cherry is smetimes stained very dark, to make the grain look more even. Many pieces of dark (almost black) furniture sold as 'cherry' are actually alder with the dark stain and called 'cherry stain'.

As for the warped top---if it is fastened securely, taking it off and letting it just sit for several days might allow the warp to even out. But, that might not happen---so replacing it will be necessary.