Wish it could be that way but actually the HTML for email is a more complex, obsolete and not homogeneous one. Each major email clients (and smaller ones too) come with their own rendering engine, making things super hard for developers to learn and keep up-to-date, which is the reason we decided to create and open source MJML, providing a lightweight and semantic syntax along with a component based approach making it highly extensible.

Because you can't reliably use <link>, <style>, float, flexbox, and whole slew of things that you take for granted in regular webdev[1].

[1](https://www.campaignmonitor.com/css/)

Imagine the web is different in these ways:

* instead of a handful of desktop browsers, there are over a dozen, each with their own quirks, most stuck with support for standards over 10 years old.

* similarly, mobile has the problems it has now (have you ever tried tracking down an Android rendering bug?). But a few more browsers to reckon with on top of the profusion of screen resolution/OS/browser combinations.

* Most ISPs run HTTP requests through proxies which each strips out their own set of tags, CSS, and JS they've decided are unsafe or unhelpful. This set is different by ISP, but you have to assume your page request could go through any of them.

Bootstrap probably wasn't built for that.

You're being down voted and I think that's unfair because it's always good to ask the fundamental question of "why".