There are plenty of sequencing softwares in the market suitable to each one's budget. However, it all depends on your $$$ and interest.
Why Protools, Cubase, DP and Sonar are famous ?
1. Constant R&D development and introduction of new features
2. Stable and strong MIDI
3. Easy and user friendly
4. Good codecs for super audio quality
5. VST Plugins and compatibility with other softwares like Reason, Battery etc.
6. Adaptability to any soundcards (except Protools)
7. Editing features during post production stage
8. Effects and softsynths
9. Sample loading and editing capacity.
10. Speak with old as well as new MIDI units (Keyboards, Modules...)
The above companies have been really working out to be the best in the market..Hence they are the top ones..Also they provide excellent customer support and training.
This is my simple explanation..You need more, catch me up at SOFT meeting on this saturday..
Cheers !