You need at least one <p> tag around the text, that you want to see in Reader View, and at least 516 characters in 7 words inside the text.
for example this will trigger the ReaderView:
<body>
<p>
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
123456789012345678901234567890123456789012345678901234567890123456789012345678901234567890123456789
123456789 123456
</p>
</body>
See my example at https://stackoverflow.com/a/30750212/1069083