As others have said its simply a lack of innovation/progress.
Line 50 has changed very little in the last 20 years, its been a great cash cow for Sage. In fact ever since the 90s when their greatest success was 'accountants in practice' promotion ensured all accountants got a nice little 25% for every referral. What do you do when you start a buisiness? Go to an accountant ask what 'What do I need?'
That took Sage from just an accounting software vendor to massive scale, I was working at Pegasus in the early 90s and they were pretty equal up to this point but Sage left them in the dust. Not a better product just better marketing.
I actually wrote software that migrated data from Line 50 other formats for SaaS providers to eat some of Sage's lunch (Kashflow being most notable) but hey being the incumbent with huge market share still hasn't really threatened their dominance. Even doing an appaling job with Sage One didn't hurt them that much,
Don't even talk about Line 500, the original Tetra CS/3 product from 80s where all the database table names are still 8 characters long because of the old DOS naming restriction when the data was stored using Btrieve!