Their support base might be limited to a specific region (e.g. the SNP in Scotland) Their electoral impact might be limited to one election (e.g. UKIP's vote collapsed in the 2017 general election)
The first-past-the-post electoral system is not kind to parties whose vote is wide but shallow, rather than concentrated UKIP won nealey 13% of votes in 2015 election but only gained one seat