The Bulgarian legislation have set a restriction for buying land by foreigners directly. This was changed when Bulgaria was accepted as EU member state in 2007. However, the accession treaty allowed Bulgaria and Romania to extend the restriction for few more years. Naturally both states took advantage of this option.

The right for extending the restriction is set in Annex 6, part 3 (movement of capital), point 1 to the Treaty for Accession of Bulgaria and  Romania to the EU.  The annex to the treaty for accession provides the right to Bulgaria to keep up to 5 years the restriction for the right of foreigners to directly own land in Bulgaria:

Notwithstanding the obligations under the Treaties on which the European Union is founded, Bulgaria may maintain in force for five years from the date of accession the restrictions laid down in its legislation existing at the time of signature of this Protocol on the acquisition of ownership over land for secondary residences by nationals of the Member States or the States which are a party to the European Economic Area Agreement (EEAA) non-resident in Bulgaria and by legal persons formed in accordance with the laws of another Member State or of an EEAA State

Even longer -  7 years - term is set for owing agricultural land and forests:

Notwithstanding the obligations under the Treaties on which the European Union is founded, Bulgaria may maintain in force for seven years from the date of accession the restrictions laid down in its legislation existing at the time of signature of this Protocol on the acquisition of agricultural land, forests and forestry land by nationals of another Member State, by nationals of the States which are a party to the European Economic Area Agreement and by legal persons formed in accordance with the laws of another Member State or an EEAA State. In no instance may a national of a Member State be treated less favourably in respect of the acquisition of agricultural land, forests and forestry land than at the date of signature of the Accession Treaty or be treated in a more restrictive way than a national of a third country.

Luckily the 5 years period expires next year -01.01.2012, so the restriction should be removed. You can read the full text of the annex VI for further details.