Hello,
Outlook usually makes use of the format in original message sent you by another user. Suppose, someone sends you a plain or Simple Text message, Outlook prefers to reply using similar plain Text which does not support any Formatting facilities.
Well, I believe it's a good way to reply a sender in the format they used while sending the message.
If you wish to change the format, click More >> Move >> Other Actions and choose Edit Message. You will then see the Format Text tab and you can use formatting features like bold underline that you wish to apply.

Hopefully, it will help to resolve your problem.

Thanks & Regards,
Anjlua