You can find various threads where @martin (developer) has discused that various times as to the why.
- Component which supports HTML editing [added in 2.2]
- Editable html (beyond custom code) & CSS commenting - #5 by martin
- Edit Text in the HTML window - #8 by martin
Once you do so it disrupts the programs abilities. So even when using the external editors feature the ability is disabled for the same reasons.