AptX and its HD version (which is basically the same algorithm configured with a different set of parameters) are very simple compared to e.g. AAC. If I remember correctly, aptX was initially designed as a computationally efficient ADPCM-based telephony codec. It still is, but it's being marketed as "audiophile-grade" nowadays. As to why it's still popular: SBC was indeed suffering from unnecessarily low bitrates and noticeable low-pass filtering, so aptX did sound nicer than many SBC implementations. But I guess "sounds better than broken SBC" doesn't sell as much as "indistinguishable* from high resolution audio".
