You have to add NumberStyles.AllowDecimalPoint too:
Decimal.Parse("2.09550901805872E-05", NumberStyles.AllowExponent | NumberStyles.AllowDecimalPoint);
MSDN is clear about that:
Indicates that the numeric string can be in exponential notation. The
AllowExponent flag allows the parsed string to contain an exponent
that begins with the “E” or “e” character and that is followed by an
optional positive or negative sign and an integer. In other words, it
successfully parses strings in the form nnnExx, nnnE+xx, and nnnE-xx.
It does not allow a decimal separator or sign in the significand or
mantissa; to allow these elements in the string to be parsed, use the
AllowDecimalPoint and AllowLeadingSign flags, or use a composite style
that includes these individual flags.