In Insert Symbol, set your coding type (at the bottom right) to
Unicode and go to the Mathematical Operators section (dropdown at top
right), where you'll find Therefore at 2234 and Because at 2235. To
enter these in your text without going through Insert Symbol, type the
4-digit code followed by Alt+X; if you're going to use them a lot,
then assign a Keyboard Shortcut to them when you're in the Insert
Symbol panel.
This avoids the legacy Symbol font, which can cause strange
compatibility problems.