Artie F and the OP are correct to classify Jet2 as financially stable. As part of the Dart Group they continually turn a profit. To say investors don't "like" DTG is rather harsh; whilst DTG provides a continuous return they do not pay high dividends to shareholders, preferring to reinvest the money in the company. Investors looking to turn a quick buck therefore go elsewhere.
I'd hardly say they were standing still either - the "Jet2holidays" brand in particular has seen considerable growth over the last 12-18 months.