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Why was impure amorphous boron the only known form for more than a century?

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A labeled diagram of the periodic table entry for boron shows its atomic number 5, symbol B, electron configuration, atomic weight range, and classification as a solid, weakly acidic, other nonmetal with a rhombohedral crystal structure at room temperature. — boron Properties of boron.

When boron was first isolated in 1808 by Joseph-Louis Gay-Lussac, Louis-Jacques Thenard, and Sir Humphry Davy, the resulting product was an impure, brownish-black amorphous powder. This form remained the only one known for more than a century because pure crystalline boron is exceptionally difficult to prepare.1

Achieving the pure crystalline state requires the reduction of boron halides, such as boron bromide or boron chloride, with hydrogen on an electrically heated tantalum filament, a complex and demanding process.1